I'm currently giving a try at leaving my beloved n900 behind with an S3. The hack-ability of the n900 was fun to have but it wasn't something I relied on. I actually like the n900 as a good phone. The real motivation was to move to more modern hardware ( better radio, better battery life, some extra CPU , etc.) but the biggest reason was to be on a supported platform. Any apps or such that come out are either Apple or Android. Things like work apps and latest apps of any type were off-limits on the n900. But, so far the S3 doesn't quite hit the same buttons as the n900 on just plain smartphone usability. The s3 was a compromise for price vs functionality, given I was going to loose features no matter what I paid (why pay for the top-of-the-line when it's still not as good as I have now?).
Things I liked in the n900 that are missing: Landscape mode as default (I prefer this, Android has it as a late add-on so not supported everywhere). Hardware keyboard (just no getting past how useful that is). Cover for the camera lens (protects it form scratches in a pocket or pouch). Instant access to camera (tied to the above cover, flip it open and camera starts automatically. great convenience). The plugin nature of the contacts app (you can download additions to it to increase the kinds of contacts details it can handle without jumping to another app) Sound worked better (android seems to get confused when jumping between earphones, speaker, Bluetooth when routing sounds. Alerts can go to speakers even when earphone plugged in, system sounds don't always go through Bluetooth when it's connected, can't reliably make it silent since some things will override that), and mostly just being able to change something if I didn't like it. I guess that's a vote on the hack-ability of the n900 but I did the minimum just tweaking things that I didn't like. Another more "fuzzy" thing is just better interface: seems like I'm having to do more clicks to get anywhere in Android. The N900 seemed well thought out and very consistent across the whole phone. Oh, and what is the issue with not including text notes by default? As a PDA veteran, taking text notes is a must for a portable device, yet seems like it's been forgotten except for add-on in most modern devices. And I like the X based interface...can't count how many times I forwarded an app to the phone in a pinch to get to something on my PC. I'll miss that
Things I have now: better connection to towers. seemed like the n900 was having more trouble talking to towers as time went on with more drops, less bars on average, etc. Supported platform: can buy accessories and find software more easily. I am supported with work and other apps. Much better battery life, although I had no issues plugging the n900 in each night, but it's still nice to get 2 days on a charge. One of the nicer things is support for MMS built in: the n900 had an app for it but it maintained separate menus and interface from regular messaging. That was another work related convenience that pushed me.
So, I'm making myself do without some things just to get the few important (mostly work related) features I consider critical for future.