I guess it depends on the place you stay at. I haven't been at Sunny Beach since I was a child but two years ago I went to Golden Sands (also a famous bg summer destination) and I had such a great time!! I live in Tenerife but I swear that the Bulgarian coast is so much more beautiful! The sand is soft and white, the sea is chill, stunning nature, great clubs, good treats, amazing food......!!! I read somewhere here that the nightlife is "cheap". Well I went to places which you can't find in Tenerife referring to music, quality of the sound and athmosphere (the clubs were called PR and Arrogance, just for the record).
However don't forget that you're still in ****ing Eastern Europe. You'll find gypsies everywhere, people won't be nice to you, if you get drunk you'd probably be robbed when you wake up the next morning on the street, your hotel room service will suck and they'll be trying to squeeze money from you second after second. Just remember to avoid problems by not trusting the staff. xD
But it's still a great place to visit. And if you want a little bit more than parties and beaches than you have some spectacular cultural heritage. Bulgaria has a lot to offer at every level, the matter is that they fail to promote it. Turkish and Greek agencies really know how to sell themselves on the other hand. xDD
My suggestion is going to Golden Sands instead...