I have a "Change Up" and am surprised to find that it is my most used bag out of an extensive (too much so
) collection of Think Tank and Domke bags. I like it because it is pretty slim as far a belt bags go.
I use mine without the padded insert which makes it more roomy and less bulky. I usually carry a 5D-II with 24-105 and a 70-200/4 in mine with occasionally a flash and I usually have a bottle of water in the outside-end mesh pocket. I find it more comfortable for me to have one of the large sections of the belt deployed as the shoulder strap attach point when using it as a shoulder bag (the web-belt section tucks into the large part so it doesn't just hang out). I can also then have a small modular bag attached for a P&S, cell phone, and other stuff. If it gets too heavy on my shoulder, I simply deploy the rest of the belt and transfer the weight to my hips, using the shoulder strap to keep it up while I walk (too much gut ... not enough butt
).
I find it the perfect size for long-day walk-around when traveling, and it carries just enough for a given outing. It also packs easily in my carry-on luggage where I usually have some additional alternate lenses for special occasions which can be swapped out depending on the nature of day's activities.
I had a "Speed Freak" which is the mid-sized bag in the Speed series you are also considering. I liked that it had a large wide-open top for easy entry, but I found it a bit bulky for me as a belt bag, so it never got used and I eventually sold it. They are really nice bags as well, as are the Multimedia series (I have a Multimedia 20 ... which alas also hasn't been used yet
), but are much bulkier than the Change-Up. I also am using the Retrospective 10 and 20 as well as the Domke F-5XC and F-5XB, all of which I am finding useful.
Hope this helps with your choice...
-howard