Last year I played the Titleist Pro V1x. Once all the snow is gone and the courses open I'll experiment with the new 2011 Pro V1x. I'll probably end up switching, the 2011 Pro V1x is supposed to fly a little higher, and that would be perfect for me.
I like almost everything about the Pro V1x. I find it flies straighter than the Pro V1, without losing the ability to shape shots and manage trajectory. I also love the flatter, penetrating trajectory. Around the greens, it releases a bit more than the Pro V1, and I find that it is more predictable in reacting (check up vs. release vs. skid and stop vs. spin back) to whatever shot you are trying to hit. In my experience, the Pro V1 tends to spin a bit too much for my liking. The firmer feel and sound off the club appeals to me as well. The only thing that I would improve is the durability. After a couple wedge shots, the cover is shredded. In tournaments, I can't play the same ball for more than 4 or 5 holes before having to switch (if I'm just playing a casual round I don't care, I've played the same ball for a couple days). The 2011 Pro V1x is supposed to be more durable, so it should fix this issue.
I haven't really tested any other balls; I would admit that I may be blindly loyal to Titleist. I've tried the Callaway balls, but couldn't stand the look of the dimples and the feel and sound off the club. I've heard lots of good things about Srixon and Bridgestone balls, from the Tour, my buddies, and also on forums like this. I might do a bit of testing with those brands. But it's going to be tough to replace the Titleist Pro V1x in my bag. Gotta trust your Titleist.