I was a (Sony-)Ericsson fan for the longest time, until I had the misfortune to buy one of their Symbian/UIQ phones, which I put up with for six months until the G1 came out. I had high hopes for Maemo (as an even more open development platform than Android), but I think the Meego merger pretty much has killed it.
My perspective on Symbian is that they ended up in a bizarrely contradictory place where their system was insanely hard (by modern standards) to program for because they still had the "embedded, must be able to run in 256k" mindset, yet somehow they didn't have the screaming performance that ought by rights to go along with the ridiculous over-optimization they were claiming to do. My SE M600 had 64x the RAM and 26x the CPU of my 1985-vintage Amiga 500, and was still slower. And the Amiga was a real computer in its day with real development tools. For some reason we'll probably never learn, the folk at Symbian apparently failed to notice that the platform had changed under them, and not surprisingly got their arse kicked by someone (Apple) who did.
Oh, and for my money Windows Mobile 7 will fail. MS's vision is "a computer on every desktop and microsoft on every computer" and, frankly, that's the only thing they've ever done well at. Every foray into other markets they make has failed - at least, by comparison with their desktop near-monopoly - and I see nothing sufficiently compelling in WM7 to think it's anything other than more of the same