We stayed at the Hotel Tirrenia for a week from 16th Sept and had a great holiday. The hotel was excellent, Viareggio was varied and interesting - as well as the beachfront there's a thriving port and local fishing boats, there were buses and trains in easy walking distance and the weather in mid-Sept. was fine and sunny but not overpoweringly hot.
The Tirrenia is a small, very stylish, family-run hotel where everyone was very helpful and friendly and the standards of room, breakfast and service were excellent. We had a first floor room with balcony overlooking Via San Martino and a view down to the main boulevard and sea. The street is busy and can be noisy during the day but it's in the heart of town and we enjoyed sitting out on the balcony watching the world go by. There is a small supermarket round the corner and plenty of good restaurants close by with a recommended list supplied by the hotel. The private beach is a short stroll away: you pay to get in and for chairs and umbrella (~14 euros I think it was for two of us) which includes a discount as hotel residents: the hotel supplies a beach bag and towels. Luca was most helpful in lending us a guide book and map, a plug adaptor for our kettle, and cups.
Pisa, Lucca and other parts of Tuscany are within reach by train or bus. We got the bus from and to Pisa Airport: a cheap transfer with a single fare of 3 euros for a journey of just under an hour. Just make sure at the airport that you spot the right bus: it wasn't at the advertised stand and we were lucky that a local Italian boy found it and shouted to ask if anyone else was going to Viareggio just in time for us to get on before the bus left.
We thoroughly enjoyed our week and would love to go back and stay at the Tirrenia again.