I marshaled at the Isle of Man TT for a couple of years and although the first year was great, the second year was varying degrees of misery!
There wasnt much in the way of rewards, but thats not really why I wanted to do it. We got a free programme (worth about £15), free pint token and a marshals dinner (with 2 pint tokens!). I think there is now a system of discounted ferry tickets if you marshal 2 consecutive years, but not sure how that works as it came in after my time. Oh and you can keep your orange vest that has "TT Marshall" printed on it! You can also sign up for a full day first aid course which is very worthwhile.
They have a good online system for booking your shifts and locations which seemed to work really well (though with the proviso that if there were too many in one place and not enough elsewhere you might get moved around).
I think they need around 550+ marshals in place to run a session (its approx 37 mile course and all posts have to be within line of sight of the next ones either side), so over 2 weeks thats a lot of bodies and they do sometimes struggle, particularly with the first practice sessions. There also seems to be an issue with people signing on to marshal, taking the freebies and then not signing on shifts or turning up to actually work shifts. You wouldnt think it worthwhile with those kinds of rewards, but everyone loves something for free!!
On the plus side you can sign up for some of the best parts of the circuit that arent open to the public but on the negative, there is an awful lot of hanging around with nothing happening and being stuck with the closed roads (though that is the same for spectators!) .
Overall I am glad I did it for the experience, but I wouldnt rush to marshal anywhere again.