I have used Ramble on my 7100g and never had a problem with it. Being that I'm coming back to BlackBerry from using a Treo 650, I just switched my Verichat for Palm OS license over to a Verichat for BlackBerry license. My fee was prorated for the BlackBerry license. If you're not satisfied with Ramble, you can always try Yak-On, IM+, or Verichat for BlackBerry.
Point your BlackBerry browser to
www.yak-on.com/download. It has a 15 day trial and a one time registration fee of $19.95.
Verichat for BlackBerry
www.pdaapps.com $34.95 yearly registration fee.
IM+ for BlackBerry
www.shapeservices.com not sure of the costs for registration.