Off-peak, there are 4tph of stoppers London-Reading, though they don't all stop at the same stations, leaving 2tph to most of the smaller stations, with all the stoppers calling at Ealing Broadway, Slough, Maidenhead, and Twyford (I think). 2tph generally continue to Oxford. In peak times there are a few more trains and most of them stop everywhere, plus there are a few fast trains to or from Slough, Maidenhead and Twyford (there are lots of commuters).
Heathrow Connect obviously provides another 2tph from Paddington to Hayes, and then there's 2tph to Greenford too.
You'd be an idiot to get one of these if a fast train is available - you're talking over a hour London-Reading compared to 25-30 minutes!