4
As Lance says the disadvantages are the core files, but it is well supported.. and the last 3 XOOPS upgrades, i released updated core files within a day of the new XOOPS version being released.
there are some hacks that may interfere, mainly gijoes autologin hack and marcans multilingual hack, but if you're comfortable with editing then you could apply the hack to those files manually.. it's the same anyway when new XOOPS released, you have to wait for updates for those hacks too..
the theme with a little time and patience can be made to match XOOPS theme using ipb's built in css and skin editor..
take a look at
http://www.bbtb.co.uk/modules/ipboard/ for a demo. (it still needs a few tweaks here and there)
advantages in my opinion far outway the disadvantages, i think it's faster for 1.. but if you don't require all the features then you have to decide whether it's worth it or not..