I use Zerojay's Jabber server (jablet, I think it is called) and it works perfectly, enabling the built-in contacts app to work seamlessly with all my friend's and family's various im services. It's one of those great things that I simply set and forget. It works.

Gizmo, imo, is a dud. Basically it is just another service that requires it's own client (so unless I convince everyone to download and use it, right it is a 'walkie-talkie'). Gizmo is also pretty 'fussy' about even simple voice connections (at least for me, on my simple, unsecured home wireless) and if you can get the video to work, which I did not, consistently, at least, as another poster mentioned, it is a worthless borg-like image that makes me realize I never really wanted video calling in the first place.

The one thing that Gizmo might be useful for is the fact that having an account seems to 'automatically' assign a SIP phone number for me to use. Of course, there certainly ways to obtain a number without subscribing to Gizmo, but that's the extent of my knowledge

Skype, well, I haven't been able to get it to work on my N810 yet, but honestly I also can't claim to have tried very hard. For me, IM is must-have, but VOIP is not (yet).