I just came back from Mexico in May and it was fantastic. There was alot of holidaymakers there and it was really lively. I think thats probably because the Americans have their spring break in May.
I've been told not to go during June-Sept because of hurricanes and bad weather. We're booking again for later on this year. But I'd say anytime between October - June is good to go

I'd definately go back in May because its so lively

Hope this helps.
Savannah