I don't find TB3 to be that bad...
...once you've finished poking it.
First off, on a netbook, the decision to get rid of the minimal header banner sucks to hell and back again. I mean, why the hell do I need to see, on screen, that the message was sent to ME? [if I suspect something, ^U to see the message source is more useful] And why do I need to know if I've <star>ed it? And given the date/time are in the message listing just above, why repeat this? Thankfully there's an add-on to restore this so the header says what is important (subject, from) and not a lot else.
Secondly, what the hell is up with the indexer? On a fairly new install with myself 500 messages on an eeePC901 (Aton N270), the indexer indexed... and indexed... and indexed... and indexed... and consumed time and CPU cycles and lots of both. I turned this off, can still search for stuff in messages - so what was the point?
Thirdly, there is a lot of stuff to be altered. I suppose, for IMAP, the "download everything all the time" option more or less breaks the point of IMAP, leading you just raise your arms and shrug.
Then there are the bugs. Oh, the bugs. Like clicking on a search result opens a blank window (helloooo? wtf is my message?). Like how the status update seems to update AFTER it has done something, and there is nowhere near enough clarity in the status - I have six POP servers defined - so when you say I'm connecting to a server, um, which? Like how sometimes TB3 will refuse to fetch new mail on an account with a message like "This folder is currently being processed", which is likely not true as nothing seems to happen (quit/reload works).
But for all of this I still use TB. Why? It seems, to me, to be one of the better programs, but there's a way to go yet...!