I have no major gripes with it or anything, but quite a few of the games that use it are, pretty much, shit. This produces a bias at first. The main difference I really actually care about is the graphics, because good gameplay can basically be mimicked by any engine.
Any modern engine with at least half a ball can boast all these fancy graphical techniques such as specular and realistic shadows and all that. However, with every Unreal game I've seen, these neat features are left out. This is most likely due to optimization for consoles and such, but also runs more slowly on my computer than, say, Wolfenstein, which uses specular and fancy shadows. I'll admit the depth of field sucks so I'll have to give credit to UE3 if that makes you happier (but the best DoF award goes to STALKER: Clear Sky).
I'd also like to speculate that the Doom 3 engine has received much more graphical mods, which, as far as the evidence I've seen, is not possible in the UE3. A prime example of this is shown here.
http://doom3.filefront.com/file/DentonMo...lpha;94630 This is not fully optimized to work on very many video cards, but it's a mod, not a game.
I may be completely wrong and I've just never seen a graphics modding community for a UE3 game, but I need proof om nom nom.
There are areas in each engine that excel differently. Each to his own. Developing on one or the other engine would probably just boil down to which engine you're better at using.