I use Psiloc Xelnex, which is generally very good, but has an annoying "feature" of starting the warning beeping if you get a calendar reminder, SMS, email etc whilst in a call. Psiloc say it's a Nokia bug and they can't do anything about it.
I'd advise testing them and see what they do if you get an SMS/reminder during a call.
Any thoughts on this app vs. Ultimate Voice Recorder?
If anyone's interested I did a quick test of both apps (trial versions) on my E71 and ended up going with Ultimate Voice Recorder -- it seems a little more polished (like fewer spelling errors in the options menus, if that's important to you)...