I use it - LOVE IT! I have a Curve and have reprogrammed the voicemail access number to the YouMail access number, so checking my voicemail works EXACTLY like my regular voicemail.
I tried it for a while and it worked ok at first, but eventually became worthless. after about 2 wks the text transcripts stopped working, then the next week all the texts stopped working. soon after, about 50% of my calls were no longer going through to my phone, instead they were immediately sent to my YouMail mailbox, but with no text alerts and since your native blackberry voicemail notification icon doesn't work with YouMail, I had no idea I even missed the call, until I would check my voicemail and see i had 18 messages and only 2 missed calls. i eventually cancelled, and even that was a hassle, they are great telling you how to set the service up, but if/when you cancel, they treat you like a jealous ex girlfriend and don't tell you anything. had to search CB to find out how to set my voicemail back to normal.
All in all i think its a great idea but poorly executed, until the service gets better I would even bother.
I have used it for a couple of months and love it also. Very useful app as I can have vm for my business callers, ditch blocked calls and telemarketers and custom greetings for my friends.