I remember when he was first announced to play the young Obi-Wan around 1997 and thinking: "the Trainspotting guy? How's THAT going to work? He's nothing like Alec Guinness."
Turns out he did quite a superb job both paying tribute to Guinness as well as forging his own interpretation of the character as a younger man. Ewan and Ian McDiarmid were clearly the best things about the PT.
I thought all of the actors did as good a job as possible with the material they were given - except possibly Hayden, whom I have never cared for as an actor. Even in better movies, his voice sounds too flat, and when he does emote it is way over the top.
Probably in the minority here, but I'd not care to see side stories in the SW universe. Star Wars movies have always been epic, and dealt with galaxy defining events and a movie with a smaller scope simply wouldn't work, IMO.
Of course, I could be way off. If there were a sufficiently compelling character I wouldn't mind seeing a film focused on him or her. For example, a Han Solo movie when Harrison Ford were in his prime would have been great. But until the new movies come out, we won't know if there will be any characters worth making movies about, and there certainly any in the prequels I'd care to see in their own film.