I have a Slider, my wife has the CDM8900.
I love my slider:
Good: It's small, sleek, has a GREAT speakerphone, my battery has lasted over 3 days with still a good charge, it has good range, it supports MP3s (modified) and MIDI ringtones.
Bad: I used an external antenna and had to have my phone replaced. It's a digital only phone. The screen is small and unprotected.
CDM8900. My wife has only has it for 1 week, so I have limited experience with it.
Good: Large screen, 640x480 res camera w/ shutter control, large menus, buttons are easier to use, you can DL the pic's to your computer for free w/ a cable (Slider and CDM8900 can use same cable), battery seems to be good, it's a Tri-Mode (analog and digital), it works great w/ an external antenna, it has an outside display.
Bad: Speakerphone is not very loud and distorts when turned up. If you want to SEND the pictures to another phone, it's expensive (w/ Verizonbut anyway.. but you can still DL to your computer for free). I like the phone a lot though. It's only slightly bigger than the Slider (actually, it's a tad thinner).
I am still glad I have my slider. It's a matter of opinion. I'll admit, it'd be nice to be able to take a bunch of pictures with just a push of a button.
Hope that helped. Again, I've hardly used the CDM8900. I only told you what I've noticed.