I'm also planning on returning it, mostly because of size and weight. After using a Focus (original) for the past 15 months (and a 3GS 18 months before that), the 900 feels like an anvil.
I made _several_ trips to the AT&T store before I purchased the phone. I held it in my hand. I walked around the store with it. I sat it side by side with my Focus. I even asked the associate if I could put it in my pants pocket to see how the size/weight felt. It all seemed fine and acceptable...until I took it home and started using it all day, every day.
I have average hands; and as a frequent one-handed phone user, I find it difficult to keep a grip on this thing. Honestly, I don't know how mere mortals can manage with these phones as they get larger and larger. The size, combined with the super smooth polycarbonate body, means that this phone WILL be dropped. Many times.
While the squared corners may be a "bold" design, one of these days they are going to tear through my pocket. Also, these corners can do a number on the palm of your hand, if you talk on the phone for any length of time.
I've also come to feel that larger screens are wasted space on these WP7 devices. Tiles that seemed reasonably sized on the 3.7" Focus screen seem clownishly huge on this device. Don't get me wrong, I LOVE the Metro UI...but oversized text on a phone like this is downright oppressive.
The downside to all this is that if I want a WP7 device with any sort of decent hardware or storage (more than 8GB), I have to go with a larger device. This may push me from the platform entirely (and back to an iPhone).