I haven't been in 5 years but did 3 years back to back and loved it.
Weather will play a big part in your experience. Wet, you'll enjoy it but can get frustrating sitting in the cold and riding in the wet.
If its sunny, game on. Racing is epic and make sure you get out on the mountain nice an early for some clear runs before the traffic builds. Only issue is there are less and less delimited zones during TT to minimise accidents.
Viewing points, you have to do Bray Hill, Ballaugh bridge and Sulby Straight. Lots of other places but be prepared for them to be busy.