My bar preference is Bongos, since all I really want are resizeable movable bars. I tried Trinity Bars for a while and dislike the options setup. It was far too confusing and the methodology to choosing bar size, transparency, button arrangement, etc were inconsistent and far too complicated.
Trinity was my bar mod of choice until a GUI was incorporated. Now I can't seem to get it to configure the way I want at all. I tried Bongos, then tried Bartender3, and I've settled on Bartender3. Bartender3 just seems the most similar to the old Trinity, being able to turn on a "Configuration" mode and just drag and dropping the bar to where it should be (this very may well be part of Bongos, and I was just too much of a doofus to find it).
I actually went the opposite way of a few people in this thread, going from Bar3 -> Bongos. I like how it's much easier it is to configure, as well as bars snapping in config mode. My only question (to slightly derail) is that I got the first working bongos as soon as 2.1 came out. Question is, does the petbar work on any newer versions? Unfortunately, my pet bar isn't working, which makes playing a hunter a little more annoying, hehe.
I used to use Bongo's, then as many here have as well, went all Ace. Now, I am back to Bongos. Ace is great, but Tuller writes some lean code, and the minor (if any) performance improvement I was getting wasn't worth the features and usability I was sacrificing.
For those of you talking about Bongos, make sure you're getting Bongos2. Definitely a big difference there. The original Bongos is still around, and buggy as all heck. ^^
Personally, I'm a fan of Bongos2. I've tried Bartender3, and I'm considering switching to it simply because of the skinning options (CyCircled stuff), but Bongos2 is very easy to use and porting bar setups across characters is a cinch.
I use Nurfed actionbars, which are very configurable.
The bars can be a little tricky to set up until you understand how they work, but they do pretty much everything I could possibly want. There are though still one or two minor issues due to the way they work (my "biggest" issue being that you can't see if on-next swing abilities are queued as they don't light up).
I used bongos2 since March, previously I used Bartender3. Now I'm back to Bartender3 because I felt like bongos2 were giving me some "casting problem". I'm not totally sure that was its fault, and not a lag problems, but it often happened to me to press an hotkey and no cast was put "in queue" (i mean, it was like I hadn't press it at all).
For customization i like bognos2 the most. Bartender3 doesn't support keybindings specific for every character, has a sucky paging system (it doesn't switch bars permanently, it works only when the switch key is on-hold, and that's useless to me because I can afford to display 4 bars, so I hotkey stuff in bars 2 with ALT and 3 with SHIFT (CTRL sucks a bit for keybinding, imo)... I want to switch my bars completely because of a change of gamestyle due to a change of the enviroment (BG, solo PVE i.e.)).