I was there last year, particularly for the first Star Wars Weekend (May 16 - 19), so I spent a lot of time at MGM. On Friday, it wasn't that bad, just REALLY busy by Star Tours and Tatooine Traders. Also, the special WWTBAM were PACKED. The rides did not have horrible lines. On Saturday, the lines were a little better, with a lot of people waiting in lines for autographs and such. We went to MK at night for an eride and I would say that the lines were okay. We did Fantasyland during Spectro and were in Frontierland for the fireworks. We waited line 5-10 minutes for every ride. Not as bad as a summer eride, but I have been to ones with no waits. Went to MK and MGM on Sunday and it was fine, lines not that bad. Monday we went to Epcot and it really was not busy at all, even though it was still the Flower and Garden Festival.
All in all, the worst lines that I waited in were all for the Star Wars events, so the crowds really were not an issue on the normal rides. Another thing that goes on earlier in the month are the grad nights, so you may want to check those out so that you don't end up at MK on a day that it closes early because of that.