I haven't used a rail pass but travelled on the TGV though they have 1st & 2nd classes and both are great. The TGV trains make anything in the UK feel like they are from a 3rd world country.
They are roomy, comfortable, quiet and air conditioned.

Please be aware that you will need to buy your Railpass in the UK as they are not available to French residents. Bonnes vacances!

I haven't used a french rail pass either, but I did go interrailing a few years ago. It was largely good, but you had to pay extra for the fast trains (TGVs) and to be honest it wasn't worthwhile gong on the slower trains. I felt it was a bit of a con that we had to pay each time we wanted a normal train.

You have to get a reservation for TGV's, international high speed trains (such as Thalys, Lyria etc), and sleepers. I used a 4 day France railpass back in January. I went on 9 TGV's which were 3 euros each, 1 Corail Lunea from Monaco-Paris (19 Euros for a 6 bunk couchette), and a Thalys to Brussels (42 Euros one way). You can get these at major train stations, or the Deutsche Bahn website.

I think the France Railpass has been discontinued though, and has been replaced an Interrail or Eurail One-country passes.