Been to Mastihari a few times, the beach  is huge but can get covered in seaweed near the town end at certain times, but if you walk further along it clears up, but be aware that the far end is used by nudists as well. The Achiles hotel is a huge complex around a very large swimming pool, which gets crowded since the place is mainly all inclusive ( not my cup of tea). You can walk into the town along the beach path which is paved.I think  the Mastihari Bay Hotel near the town is far better.
The town itself is fairly quiet with a number of bars and tavernas, for more night life go to Kos town ( and for shopping)  Kardamena is the tourist town - 18-30 chips, burgers and lager.
There is a large waterpark just outside the town, so if you have kids they will love that.

I will definately be returning as I think this is a lovely place