We have booked this hotel through www.laterooms.com and have read a few reviews about it on tripadvisor as well. It looked pretty well and it was reasonable enough for late August. We payed €105 per night and that includes breakfast.
The location is superb. It's only a few minutes walk from San Marko and the Rialto Bridge. The directions given on their website (www.casaallafenice.com) sound funny but will bring you right there :)
Inside the hotel you will be amazed. The decor is amazing. They have certainly spend a lot of time and effort to make it look this way. The staff is super-friendly and very helpful.
The room itself was a bit on the small side but spotless and prettily decorated. The bathroom (ensuite) was big and very clean as well. The breakfast is not great but it is brought to your room...wow! You can specify a time when you want to have it and you can choose between tea, coffee, hot chocolate, milk and juice.
I would highly recommend Casa alla Fenice.