I certainly like Q. Well, TNG Q.
I've seen his DS9/VOY episodes a lot more recently, and every single one of them is shit, so unfortunately that's my lasting impression of Q, at least until the next time I watch through TNG.
Obviously his last appearance in TNG was pretty damn good. Looking through the list of his TNG episodes, I'd call half of them good, and the rest pretty bad. All of his other appearances are awful too.
I just hate the concept of the character in general though. Omnipotence and power is always handled in such a heavy handed and narrow minded way in TV shows and movies. When you consider the scope of time and space a being like Q covers, his appearances in Trek just make little sense to me.
Well, that's true of TNG episodes in general.
Looking at the list of Q episodes, I'd call only two of them less than good, "Encounter at Farpoint" and "Hide and Q." Farpoint, well, it's problems are well documented and while I wouldn't blame the Q character for it being bad, he certainly didn't do much to redeem it. Also, for a race of omnipotent beings, they're pretty damn clueless about some basic things. "Hide and Q" is a typical first season episode and, as such, it sucks.
The rest are mostly pretty good. "True Q" is kinda bland, but it's largely inoffensive when compared to other truly bad TNG episodes. I'd say Q episodes have a pretty good batting average, much better than DS9's Ferengi or alternate universe episodes at least.