I never had any problems that stopped me from playing with Uplay on the Xbox 360. But even then I just found it to be slightly annoying, slower than a crippled snail, but also mostly ignorable. And last I checked it was still, after years of being in the way, since at least Assassin's Creed 1, considered a beta on the consoles.
But on the PC I just thought the damn thing was in the way all the time. Or it crashed, and messed up my game. I dislike it to such an extent that it's put me of buying more Ubisoft titles for the PC, and since that's the case they're really just doing it all wrong.
I mean they add nothing new, aside from problems. Steam already does all that Uplay does - without getting in the damn way - and I don't need another system drain having to start up before I can even launch the game, reminding me I got a damn achievement, a friend's online or pestering me with it's damn cloud synchronizations. Synchronizations that crash or fail every once in a while... and that's if the damn thing even manages to log on.