I've used ROM Manager to flash ClockworkMod Recovery onto my Motorola Droid, but when I try to reboot into the ROM, I just see the Motorola logo for a while and then it reboots again back into the phone. If I try to install a custom ROM, instead when it reboots I just get an exclamation icon and have to manually power down and reboot.
From what I've found online so far, nothing has worked as far as solving the problem. Flashing repeatedly, or going between ClockworkMod Recovery and SPRecovery didn't solve the problem, nor did reinstalling the ROM Manager application.
Interestingly, the SPRecovery ROM seems to boot fine, so I'm not sure why ClockworkMod fails every time. Trying to flash old/experimental versions of ClockworkMod always says "An error occurred while flashing your recovery." so I don't know what's going on there.