I have the M877 - and it works fine, however programming the count-down timer is a bit tedious. It lacks the nice features of the other products suggested above.
Here's another angle to consider: I have a voice recorder in my DRE Symphony intercom and I can play back recent radio transmissions. This is handy for those fast clearances - I say "Standby for readback" and then playback the recording as I write it down.
The DVR 300i clock by FlightCom has a voice recorder. I've never used it, but if I didn't have the Symphony, I'd strongly consider it:
http://www.flightcom.net/site/products/dvr.php