Hello everyone...it's been quite a while! As some of you may know, the reason I haven't posted in many moons is because I did a complete switch from my 1Ds II kit with a bag full of L glass to Micro 4/3 last year. I've been really happy with the switch, though I still use a little bit of Canon, though nowadays, it's adapted FD lenses. 
Today I got my first L in FD mount, a rather rare lens that I frankly didn't even know existed until just recently.
It's the Canon FD 50-300mm f/4.5L. It's a relatively large and very dense lens. It weighs in at nearly 4 lbs. I got lucky on this guy. As I mentioned, it's a pretty rare lens. (Mine was one of the first 500 manufacturered) It typically goes for between $750-$1100, and I managed to snag one for $550, and while there are a few minor nicks like on most old lens bodies, the glass is absolutely perfect. Not a mark on either the front or rear element and looking into the lens, I don't even think there's a speck of dust. It is flawless. Zoom and focus actions are smooth. Aperture blades are clean and snappy. The only thing that is slightly off is that the aperture ring is a little stiff near the wider apertures. It's not the aperture itself (as you can easily snap the blades open and shut with the lever on the mount without any hint of stickiness), but the actual aperture ring. I'm wondering if just sticking a little graphite in there will help smooth it out a bit. But, it works fine, so it's not much of an issue.
I'm using the lens on micro 4/3, so it's field of view equivalent to a 100-600mm lens, at a constant f/4.5 aperture (though, of course depth of field will be two stops greater for the same framing and aperture vs a 100-600 on full frame).
The lens with my GH2 (sorry for the shallow DOF...I was too lazy to go get my flash, and well, this lens was on my tripod).
Optically, I'm rather impressed, frankly. It's not as good as the very best fixed focals I've used or the very best of the modern Canon L-series zooms like the 70-200/2.8 IS II, but it's still very good throughout the entire zoom range as far as sharpness goes, and color is very pleasing. It is rather sharp wide open at all focal lengths, even with the very high pixel density of the GH2, and stopping down to f/5.6 makes it very, very sharp. I'm actually rather astounded by the relative sharpness. The GH2's pixel density is equivalent to a 64MP full frame body. My guess is that looking at this lens on a 5D II would yield sharpness on par with most any modern high-end lens.
The biggest thing, especially given the age of the lens and the fact that it's a zoom, is that is controls CA and purple fringing EXTREMELY well. There is virtually no CA at almost all focal lengths (every once in a while a very slight blue/yellow CA is visible, but is very easily corrected.) PF is very well controlled...a huge improvement over something like my FD 200/2.8. See, a lot of older legacy glass, especially relatively faster glass, has a real problem with PF and longitudinal CA when used on modern digital cameras, but not so with this guy. Bokeh is generally pleasing, with it getting a little busier the further you're focusing.
Overall, I'm very pleased. It's a BIG lens, but since on m4/3, it's basically a 100-600mm super zoom at constant f/4.5, It'll work (Yes, I know DOF is not like a 600 f/4.5 on full frame). It's definitely a monopod lens, as handholding becomes tiring after about 5-10 minutes, but that's fine. I will likely only use it for situations where I really need a long lens, and I generally won't mind the size for those types of shoots.
Of course, focusing requires extreme precision. Focusing at 50mm is actually somewhat difficult, as it's a slow lens at 50mm (how many 50mm f/4.5's do you know of?), and at 300mm on close subjects, the DOF is razor thin (I'm actually glad for the slightly deeper DOF from shooting it on m4/3), but when I do my job (which I did pretty well for the most part today), the images are quite good. I have to say, the very large zoom range makes it an awesome lens to shoot with. I was generally trying to shoot stuff between 200-300mm most of the time, but when the opportunity to back out a bit was needed, it was really nice to have leeway to do so.
Some samples from the 30 minutes I had to shoot with it today:
At 300mm, f/4.5:
At about 120mm and either f/5.6 or f/8:
At 300mm, f/4.5 and near MFD:
And my favorite of the day, at 300mm f/4.5 with bounced flash:
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