I've used the MX-500 and the MX-350. The MX-800 differs from those two by the fact that it supports more devices (20 device buttons instead of 10) and must be programmed via your computer (the others can only be programmed directly from the remote).
The MX series remotes can be setup any way you want. There are no restrictions on which types of commands can be placed on which button. The activity based feature of the Harmony can easily be replicated on an MX remote, although the MX remotes don't have the 'smart state' feature that asks you questions about which devices are on to resync if a command in a macro is missed. I use a combination of activity based and device based; ie a mode like 'dvd' has mostly dvd commands but also a few of the common receiver commands I need when watching a dvd - like surround mode, late night, etc.
The MX-850 is the replacement for the MX-800 and it includes RF (have to buy the RF basestation separately). If you don't want to have to use the computer to configure the remote, the MX-650 is nearly the same as the MX-850 but is programmed directly from the remote.
