I also recommend Yak-On and have not had any problems with it since I first got my Sprint 8703e. MSN does take a while to sign on but other than that, no actual issues with the application. It does have some drain on your battery as the default 'check-in' time for the network is 2 minutes, you may customize this for your needs.
Sprint BlackBerry 8703e
OS: v220.127.116.11 (Sprint...FINALLY!)