Stayed for 9 nights in July, great room (paid extra for sea facing, definitely worth it for stunning sunsets on the balcony!) perfect size for 2, we had room 35 which actually has a balcony off the room and another off the bathroom which is great for keeping drying clothes out of the way (each room has a drying rack which is a great extra).
Hotel is in a great location, 5minute walk to the port, supermarket, lovely town and restaurants (of which there are many and all highly recommended).
Hotel also has a sister hotel Tritone which you can get a bus to (7mins) or walk (25mins), it's quite in the middle of nowhere but has a great area for sunbathing by the pools or over looking the sea or on the beach, also has restaurant and bar, wifi etc - we went for a few days for the poolside sunbathing and beach.
Nettuno has a great roof terrace one side overlooks the sea and the other a backdrop to the green landscape of the island. Its large and when we were there hardly anyone used it. Great breeze from the sea and a small jacuzzi which actually has lovely cool water - a bit awkward if someone else is using it though as very small! Also a snack bar which opens for a few hours at lunch and I believe the evening, lovely looking food although we didn't eat there.
Staff (all beautiful!) very friendly and helpful and very knowledgable about the island and transport.
My only niggle was the wifi not reaching the rooms or roof terrace, otherwise highly recommend this hotel to