I never owned an iMP-350, '400, or '450, so I don't know how them units are.
My iMP-550 is over two years old and runs fine, though I been using the iMP-900 since 2006-02-02 and when I used the iMP-550 throughout 2005, the remote I used was the iMP-900's remote (feels better).
I feel the '550 was a better unit than the iMP-900, though. It has a bigger anti-shock buffer and an optical out (if it works!).
The only negatives about the '550 seemed to be its 2-AA weirdly-shaped battery adapter, as well as the small headphone output (only 12 mW/channel, compared to the iMP-900's 18 mW/channel).
PS: My iMP-150's still kicking and is over 3 years old...
