I've stayed there on my last 2 trips. No gym, but it's in walking distance of most places around mid Soi Bukhaow/Soi LK Metro/Diana and on the baht bus route as well - most bus's go towards Pattaya Thai though, you may have a wait if you're heading the other way.
Rooms are roomy enough and adequately furnished. They're cleaned twice a week but the whole place has seen better days. Don't misunderstand me, it's good value and everything is clean and works, but 5 star it isn't. I would recommend a room on the front side (facing the pools) as they get the breeze, also IMO, the higher the better.
The staff are friendly and helpful but do tend to close the office early sometimes, bear this in mind if all your money is in the safety deposit in there! It's the only place I've stayed where, although I saw only Thai staff, they did preventative maintenance - normally without a fallang in charge that just doesn't happen. Security is at the entrance to the estate and is usually on the ball, ID cards are checked, if a girl hasn't got one they will still let her in, but you have to sign surety for her.
I'm changing for my next trip because I want an in-room safe and wifi - Soleil de Siam has neither. Also, being a fat bastard, I dislike the 23 steps UP to the lift (23, I counted them - every time! ).
Soleil de Siam is a company that rents out a lot of the condos on the Diana Estate, they don't have the exclusive contract, you can rent from elsewhere as well. As I understand it, if someone has a condo there and isn't using it, they will rent it out for you and take a cut, how big a cut I have no idea.
Does this help?