Works really well. Only supports two phones simultaneously though - this was not clear before I bought. However, those phones don't have to be registered with the repeater so any 2 of the 6 on your base station can grab the repeater. Not a problem for me but my 685IP supports 1 land and 2 voip lines so if 3 people wanted to call one presumably would need to be on the base station directly.
Impressive range - apparently more powerful than the base station. I can get outside through thick stone walls and 90ft from the building without problems.
Quite expensive compared with the base station - but it was ever thus with bespoke add-ons - Sienmens know they have you hooked - who else are you going to buy from and I guess, to be fair, they don't sell so many.

I bought one of these as I have an old house with thick sandstone walls and no matter where I put a single base station it is impossible to get reception everywhere with a digital cordless (the old analogue ones would just work though!). I have a Siemens S100 with 2 handsets. Registering the repeater to the base was easy. After remembering to register the handsets with the repeater I was up and running. The system works exactly as designed. Walking from repeater reception to base reception causes no audible event even during calls. The set has yet to drop a call or cause any bother.

I had tried the DORO repeater - also supposed the be compatable - but that was a complete waste of money. It dropped calls without warning, and was generally a far inferior product.

Simple and effective. If you - like me - have patchy reception due to solid traditionally constructed walls - work out suitable positions for base and repeater(s) and you won't look back!