Mirissa is a little different in that the beach is pretty hard to get to, unless you are staying in a really expensive beach front place. It's back off of crazily noisy Galle Road which is nice though.
The beaches are clean. and especially in Mirissa, there is a good variety of accommodation available. We were able to find an awesome cabana for a really good price. Food however, was much harder. If you are nearing the end of your travels, or you have a more flexible budget than we do, than the fresh seafood and meals in the beachfront restaurants are probably delicious! For backpackers on a budget like we are, paying 3 times the normal price even for a cup of tea wore our budget very thin. We did find some nice, cheap and delicious local restaurants in Weligama, and one not-so-expensive place on the main road in Mirissa though, which was a great relief.
It was nice, but not what we would call beach-paradise.. But that's just us.