I went last year (the 50th year for Ducati's involvement) and thought it was waaaaay better than any motorcycle racing I'd seen anywhere else. Make MotoGP look like a bunch of weenies on a go cart track (I still dig MGP though). The racing was great, the european bikers were great, the bike shows were great, the beer was great, meeting Paul Smart for the first time last year (met him again at Indy MGP and he picked right up on where we left off in conversation), sidecars are great, vintage watching was great, etc.
The island is really a wonderful location with a facinating history. Plenty of time to explore during off days (they only race every other day). However you manage to do it, take a few laps around the mountain course and you will understand my statement about TT vs. MGP. If you can stay for it, make sure to go watch the Billown Races at the southern part of the island the Sunday after the final race. Two stokers at their best.
I planned it all out like 1.5 months beforehand and used frequent flyer miles for a free R/T, stayed a few days in Dublin and did a homestay just up the street from Bradden Bridge. Get on the IOMTT website forum to find great places to stay and find out what's shaking each day.
As Brooksie pointed out, marshalls are a big part of the races there, you can volunteer to be a marshall for a few days and it would be a great experience. They need an outrageous amount of marshalls as it takes a lot of able bodies to be placed in line-of-sight of each other around a 37.5 mile course. There are some perks to doing this. More info on the IOMTT site.