Probably going to stir things up a bit with this one, but after spending a few months on Rival and even raving about it around here, I am starting to see its shortcomings. I am now thinking about switching over to Ultegra or DA. I spent a year on 105 (5600) with my Trek 2.3 before moving to Rival, and I didn't like some things about it so I was certain Sram was the answer. However, now that I have some experience with the Rival group that came on my 2011 R3, I am starting to think I prefer Shimano's shifting system.
First, the two lever shifting set up Shimano uses was never a problem for me. In fact, this is what I miss the most. I'm getting really tired of accidentally shifting into a higher gear when I try to down shift with Sram. It seems like it requires a deliberate effort in distance, force, and timing of the lever throw to accurately down shift with Sram. In other words, if you don't push the lever far enough you end up shifting into a harder gear, which really sucks when you are about to attack on a climb. This was never a problem with Shimano because the up and down shift functions were separated with two different levers. Again, I never had a problem with confusing the braking and down shift functions of the lever.
Shimano feels smoother to me as well. Everything from the shifting action to the butter smooth pivoting of the brake levers.
Sram still makes a terrific product. Definitely a snappier, more confidence-inspiring shift. 105 was more finicky as far as adjustment goes. I also like how quickly the zero-loss front shifting happens. With Shimano, it seemed like I had to throw the lever too far to shift from the little ring to the big ring up front. I've heard that the newer Ultegra and Dura Ace groups have very quick and effortless front shifting, but I haven't experienced this myself.
To make a long story short, they both have their positives and negatives. I'm just starting to think that I'd be better off with Shimano. I miss the separated shifting levers and the smoother overall operation. If the newer Ultegra or DA has improved (quicker) front shifting, I think it would be worth it for me to give it a try.