getting to some places on the TT circuit by MTB is an awesome way to see the racing and get to fairly inaccessible spots, especially up the mountain.
Second that!
Went a couple of years ago with my bro and the mountain bikes. You can see lots more of the circuit too. You could watch the first couple of laps from Kirkmichael, say, then ride round and watch the leaders come through at Ballaugh Bridge, then ride up and watch from Sulby Inn or Ginger Hall.
Between races the marshalls can let you across the circuit and you can nip across the hills in the middle, down fictitious footpaths through gorse and across streams and pop out and watch the next race from somewhere else.
If you're walking or driving, you're pretty much stuck in one place for the day. Put a couple of beers in the bag for when you're away from a pub and you're all set!