Depends what you want it for - really and truly anything is a good secondary device if it fits your specs!
I currently use a Hero as my work phone, but it's terrible so i'm going to get a WP7 phone, primarily because of the Office integration - regardless of the OS's shortcomings, as a writer having all my work documents synced to SkyDrive then to my phone is invaluable. Plus it's also an excuse for me to test drive this OS to decide whether i'll enter a contract when Nokia release their WP7 phones.

So for me that's a good secondary device, but plenty here won't consider it. It just depends what you want. Personally i still like symbian and if i didn't have the N900 i'd get one of the S^3 devices - lots of apps, great hardware, integrated social networks, great games, great battery life, fm transmitter, Symbian ticks all the boxes for me.