The main issue with an older design remote like the MX-650 is the lack of "new" function buttons. For example, there are no skip +/- hard buttons. Granted there's the screen area, but they all have that.
If you're the kind of guy that just likes to find a button blindly and hit it without thinking about what it is, the older MX remotes deliver that in spaces. The MX-450 is kind of a halfway measure between that (now visibly dated) style of remote, and the newer flat/smooth/sculpted keypad designs a la Harmony and others.
If you want some further impressions, check out my review of the URC R50: [Link: remotecentral.com]
which is the "consumer" version of the MX-450.