I got someone to move from the foldy-up seats in the bike space this morning. I did feel quite bad for doing so, but there were plenty of seats elsewhere (ie directly opposite).
I can't really comment that much as I've only been train-bike commuting for a few weeks and leave the bike at Waterloo station overnight during the week, but I've not found many problems so far with taking a full size roadie on the train. I use South West Trains from Waterloo to Basingstoke which is just outside their restriction area for bikes so it's down to the guard's discretion whether he lets you on or not.
I'm not sure whether the SWT improvements are heading in the right direction for cyclists - the older white trains have a few 3-bike racks per train but the new blue ones have 2 foldy-up seat bike spaces per 12 carriages!