Hey, have you seen this site?
it seems to be more about older cars, but still gives an overall idea...
In my experience, Altwood Taplow (now Stratstone), good at sales, not so good at service; HR Owen Chiswick (on the Great West Road in Brentford) - had to use them for some warantee work on the e39, and they just didn't want to know - basically get the job done and get rid of me. Very poor service, so bad, I ended up in Wood in Odiham
Actually got very good Sales service (the Business Manager, Noel, is excellent), although their initial p/x offer wasn't the best deal. Haven't done enough miles yet to see if their servicing is up to scratch, though.