Other than fighting with the calendar occasionally (sometimes needing to force a manual sync to see coworkers' events), Gmail in Thunderbird has been pretty smooth. I have not noticed slow search, slow mail receipt, or application crashes. In fact, the search is often too good, in that after running it it matches hundreds of more emails than I usually expect. I usually stick with the quick filter which is a bit less flexible, but generally returns more useful results.
Another advantage, on Linux at least, is that if you copy your ~/.thunderbird folder to another machine, all accounts, GUI layouts, settings, search results, tabs, etc move over flawlessly. I think you just have to re-sign in for Google accounts on the new machine and you are good to go.
I have decided to just leave it be, and let it hog my computer, because it interface decently with google calendar.
That being said, I can definitely vouch for clients like Claws Mail (a bit ugly, but does its job) and Evolution (super fast, but it's written in C#/Mono)