Hi Felix,
In response to (1), I would say that the most prominent individuals would be the Gracchi.
Regarding (2), on the one hand, Caesar and Pompey are sometimes regarded as using tribunes as their (one could almost say) "henchmen" in Rome after the death of Crassus, and one coudl argue that this is indicative that the office was of less prestige in the post-Sullan era, although I am sure one could write a PhD on the question without concluding one way or the other!
Clodius seemed to have used the office as someone of considerable ambition, and he had himself adopted into a plebeian family so that he could hold the office:
http://en.wikipedia.org/wiki/Publius_Clodius_Pulcher
The office had considerable legislative power, and one has to remember that Caesar's spectacular military career in Gaul was all about advancing his political career in Rome. City politics mattered to the Roman elite, and so the tribunate remained very important, and to Clodius, I would say it was indeed "prestigious".
Clodius seems to be mainly remembered as Milo's rival in the street violence of the time, and Milo's career as tribune does seem is indicative of less political ambition, but the office was clearly a prize worth possessing:
http://en.wikipedia.org/wiki/Titus_Annius_Milo
In response to (3), I am not aware of anything specific. You might try the relevant volume in the Cambridge Ancient History if you have access to a university library, or maybe just start with a general history of the fall of the Republic and look at the references.
The following is pretty recent with a cheaper paperback appearing soon (still on my list to read):
http://www.amazon.co.uk/The-Breakdown-Ro...7&sr=1-1#_
In addition, the relevant volume in the new Edinburgh History of Rome series is due out in the not too distant future:
http://www.amazon.co.uk/gp/product/07486...ROKL5A1OLE
Good luck with your researches!
Regards,