I'm a big fan of the Aria for several reasons, but one of the biggest (pun intended) is its size. I can carry the Aria in my pants pocket (jeans, slacks, etc) without being weighed down or having a phone bulge. Even other 3.2" Android phones are significantly thicker, taller, and/or wider. 3.5" and 4+" phones just aren't easily pocket-able. It's pretty much the smallest and thinest Android phone on the market.

Other aspects of the Aria haven't disappointed me yet. 2.1 out of the box and official 2.2 coming by the end of the year. Root and Cyanogen 2.2 running now. Wireless tether available when rooted and properly configured. Sense is extremely snappy and general performance is much greater than you would expect from the Qualcom 600 MHz processor. Sideloading apps was/is prohibited by AT&T, but a briefly available version of the HTC Sync software (or rooting) gets around that. It's a world phone. It's also cheap or free... I got mine for $55 rebate free during release week with a $75 off web promotion. When in stock, it can be had free through WireFly.

My only real complaint is typing in portrait mode can be a bit tight, but still usable and no complaints in landscape.