So far, I've always ended up coming back to Gmail for one reason or another -- while there are lots of little UI annoyances, it always gets the fundamentals right, which is better than my experience w/ 3rd party clients so far. Fingers crossed for this one. :-)
I also tried various 3rd party clients and would always end up going back to gmail's app. My dad kept recommending Spark to me and I just didn't have the energy to try another 3rd party app. I eventually gave in and tried Spark and I haven't gone back to Gmail since. That was about 18 months ago.
So far, I've always ended up coming back to Gmail for one reason or another -- while there are lots of little UI annoyances, it always gets the fundamentals right, which is better than my experience w/ 3rd party clients so far. Fingers crossed for this one. :-)