I live in a village outside High Wycombe, I cycle 25 mins to the station, 30mins train to Marylebone, then 25 mins to the office. Same in reverse. On average I probably ride 4 days a week, then if it's pissing down with rain or I'm knackered I will drive to the station then get the tube.
Pros: I do like the cycling 90% of the time
Keeps me fit(ish)
Cheaper than driving & parking
Cons: there's no denying the time is a massive ballache. I leave the house at 6.20am and get back at either 18.45 or 19.15 depending which train I get. It can cause friction at home (although my wife wanted to move out here in the first place haha.)
Train costs £350 per month. I usually get a seat but I think that Chiltern are the exception speaking to my colleagues who commute on different lines
However, having said all this, I love living in the Chilterns, it is brilliant for the kids, my house is twice as big as my old house in London etc etc. If I could wfh 1 day a week (currently in negotiations) that would be perfect. For me the benefits do outweigh the negatives (just)
EDIT also don't use a Brompton a) they are crap to ride b) everyone on the train will hate you. I have two cheapo commute bikes that I leave at each end