I don't often share my thoughts, because most of the time, others tend to speak my mind before I get the chance to voice an opinion. I'm shocked in stating that now isn't one of those times.
In my experience with cellular devices, I've found Samsung and Sony Ericsson to be by far the best companies as far as well-rounded media devices go. Nokia is an excellent company who, in my opinion, is in a class of their own, at least in reference to late model phones.
I've owned the D600 and the D900, and both served me very well. Both have decent battery life, excellent cameras, great form factor and sturdy casing, solid sliding mechanisms, and more. The downfall of the D900 is call reception. If you're in a well-populated area with plenty of towers, you'll have no problems whatsoever making and receiving calls. On the flip side, if you're on the edge of what would be a 2-3 bar zone, you may find yourself, on occasion, with only 1 or 2 bars, and although it rarely happens, sometimes you'll lose service entirely.
Now, the RIZR. I actually just bought one today, and I can safely say that aside from the actual build of the phone, it's complete garbage. Don't buy this phone, you will regret it. The outdated OS is reason enough NOT to purchase it. I'm bewildered by peoples' praises for this phone. While the casing is decent and the heft and feel of the phone are also good, it loses any points gained in all other areas other than making and receiving phone calls, which granted is the most essential function of the device, but these days, cell phones are hardly used ONLY as phones. Unless you're into the crappy RAZR keypad, low resolution screen, Motorola's terrible UI, and a flimsy feel when the phone is open, you should stick with Samsung.
The D900 will do you great justice, I can promise you that much. But, before you decide on either the RIZR or the D900, I strongly recommend you wait a few weeks for the Samsung SGH-U600. It builds on the D900 is every way possible and crushes the RIZR in all aspects a phone could.
Honestly, you'd be best going with the U600. Read up about it on GSM Arena, or do a Google search and read previews for it. If time is an issue, go with the D900, and if you don't believe what I'm telling you, feel free to pick up a RIZR. I guarantee your experience with the latter option won't be nearly as satisfactory as with any Samsung slider.
My twenty-two cents, take 'em or leave 'em,
- Tastic
