I've been using Gizmo since the E61. I've successfully used it on the E61, E61i, N95, E71 and am now (sorta) using it on the E75.
PRO: Enter in your username and password and you're good to go and all configured
CON: The Gizmo people take foreveer to accept your money and have problematic / non-existent customer support (for people who want to use Gizmo for long distance, incoming calls, etc.)
Overall: 5/5 - more SIP providers should hop on the bandwagon and create a client like this. I don't use the IM features of Gizmo but I do use the SIP calling features extensively.
PS - (as of 5/18/2009) don't install the BetaLabs client on the E75. It doesn't work - instead use the client you can download from within the phone.