Depends on who you ask. It's problematic for some on Reaper, myself included. I can get it to work by starting Reaper first, then hitting the transfer button in the plugin. It crashes (or records only a measure) when doing the reverse - which is how it works in every other host.
Celemony is cagey when it comes to supporting it in Reaper, and is the biggest reason why I switched to Studio One V2 for most of my needs. Still love Reaper, but the melodyne integration is unmatched there running as a VST3.