This first part details why I chose the RRS BH-40 and the decision making process; you can skip ahead to the next red line of text if you don't want to read it.
I've recently been wanting to get a new ball head for my tripod. I had (and still have) a Cullmann MB 6.5 head with A-S compatible lever clamp, which is a great head, but has some annoying problems: the plates which fit the clamp have pointed corners and dig into the palm of my hand, which is not pleasant. The clamp is not adjustable, which means I cannot buy more ergonomic plates from different manufacturers. The panning base uses grease/oil, which thickens and makes it very hard to pan at low temperatures. The head is quite large and heavy.
So I wanted a ball head which would fix all those flaws, while retaining the stability and smooth movement of the Cullmann (areas in which its performance is pretty darn good). This immediately meant I was looking at a much smaller range of ball heads, all of which were more expensive. I narrowed it down to 3. The Acratech GP-s, the Markins Q3T, and the RRS BH-40. I liked the design of the GP-s a lot, it looked nice and pretty cutting-edge. It would also be useful in certain situations I sometimes find myself in, where the ball head is exposed to things you wouldn't really want it to be exposed to such as dirt and lots of water. The Q3T was being considered purely because it has a very good reputation. The RRS was being considered also because of reputation, but also looks (it does look quite pretty).
I luckily managed to find a store that sold all 3 of these heads and let me play around with them (without actually mounting anything on them unfortunately). I immediately ruled out the Q3T because I didn't like the tension/friction dial at all, and partly because of the reports of the weak panning base lock.
The GP-s felt very solid in the hand, and quite smooth, although a bit behind both the RRS and Q3T in that respect. I didn't like the tension knob very much, it felt a bit flimsy. I also didn't like the petal shaped flange on the stem which meant that I absolutely had to use an Acratech clamp on the head. Finally, I didn't like that it cost more than the RRS. So I ruled the Acratech out.
Ok, now this part is the actual review 
First impressions
The head is rather short compared to your "traditional" ball head, which keeps it compact. It also has 2 drop slots, unlike any other head I am aware of. The whole thing is machined aluminium, very solid, feels very high quality as you'd expect from such a pricey piece of equipment. The panning base is clearly marked at 2.5 degree intervals. The finish on the casing is textured, and not unlike the finishing used on Canon's magnesium-bodied cameras. Unfortunately, it is more like the earlier models e.g. the 20D, where the finish doubles as a nail file which means that whenever your fingernails come into contact with the casing, you'll leave a white streak on the black surface. If you are a bit OCD about these things, then be prepared to clean this head regularly, or wear gloves/be extremely careful when handling it. You can see one such streak to the upper right of the RRS logo in between the drop slots in the following photo (tripod is a Feisol 3442).
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by noobographer, on FlickrThe head really is a thing of beauty. I am quite impressed by it.
Functionality
The knobs are oversized for easy use with gloves in cold weather. The friction dial and panning base lock are on one side of the casing, with the main locking knob on the other. The main locking knob is spring loaded and repositionable to your liking (I think 6 positions). I do not like the design of the main locking knob. Its elongated shape means that if you use it on a tripod with no centre column and a wide spider like the 3442, the knob will hit the spider. Fortunately, locking/unlocking the head does not require a full turn of the knob so it is not a problem in practice.
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by noobographer, on FlickrThe tension and panning base lock knobs are quite close together, but RRS thoughtfully made them completely different in shape so that there would be no chance of mistaking one for the other:
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by noobographer, on FlickrAll the knobs are captive, so they will not fall out if you unscrew them too much. There are numbers on the friction dial running from 0-10, but these don't really serve much of a purpose, as you can untighten it past 0, and it is extremely difficult if not impossible to tighten it past 7 or so. They just let you know if you are twisting it in the right direction.
The huge lump on the back of the casing means this ball head is relatively useless for those of you wanting to reverse-fold your tripod legs around the head. With a centre column it is better, since the legs will be able to close most of the way, but without a centre column, the legs will be splayed out at very wide angles. This was not really a consideration for me as I tend to remove the head when packing the tripod for travel anyway. It may be a problem for some of you.
The movement of the ball is smooth, but not as smooth as I'd ideally like, but as the head is brand new it will probably break in and become smoother over time. There is no sag or droop at any angle with a 5DIII with 70-200/4 and flash mounted. I am confident the head could easily hold a 100-400, though the tension dial might have a bit of trouble and you may need to use the main locking knob to fully stop any creep with heavier lenses.
The panning base is nowhere near as smooth or fluid as my Cullmann head, partly because it doesn't use any grease or oil. The Cullmann was exceptional in this area (save for the freezing up at low temps), as good as some fluid heads, so this is not a negative mark against the RRS.
***Edit*** I've looked more closely, and it seems that the BH-40 does in fact use some kind of lubricant for the panning base. I'm not too happy about this, as it likely means the panning function will slow down/become hard or impossible to use in cold temperatures, and about the fact that it is nowhere near as well damped as my Cullmann head.
I chose the B2 AS II clamp as I prefer levers over screws. The clamp is a bit large for the BH-40, which means that there may be interference if you use the drop slots with a tripod such as the 3442 with no centre column/a wide spider. However, this only happens if you have the clamp at an angle when it is in the drop slot. If you have the clamp level, there is sufficient clearance:
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by noobographer, on FlickrRRS states that the B2 AS II clamp
This means there is no need to adjust any screw or knob which if true, is very convenient. It remains to be seen how much leeway for adjustment is available with the automatic system. It works fine with my Cullmann plate which was previously tested and failed to work with a Markins clamp, I will test it with a Benro plate once I get the chance.
The clamp has 3 positions, fully open, halfway closed, and closed. In the fully open position seen below, you can lift the camera in or out:
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by noobographer, on FlickrWith the clamp in the intermediate position, you can slide the plate but not lift it out:
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by noobographer, on FlickrOnce locked, the clamp holds the plate securely without any movement. The lever is nicely contoured to take up less space and require less force to open:
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by noobographer, on FlickrThe bubble level is quite useful and is not covered by the camera.
Overall, I'm quite pleased with the BH-40, it holds my gear perfectly fine, is lighter than my old head, is just as smooth (except the panning base), doesn't have any oil or grease to freeze up in cold temperatures, looks great, has good controls, and should last a very long time. I don't see myself upgrading from this unless I decide to get a 500/4 lens.
