Originally Posted by stut

If it's a walk-up fare, you can buy whenever you like, but there are no reservations on these trains, so it makes no difference when you buy (apart from the time take to operate the machines).

How busy they are is hugely variable, although you always have the advantage that you can take a later or indirect train if you turn up relatively late to a full service. I don't know the rules about breaking a journey in Italy, though, of that's something you wanted to do.

Final point is to bear in mind that some of these trains may be somewhat, er, rustic. Not unpleasant, but certainly nowhere as clean, modern or well maintained as the Eurostar Italia services.

However, I've used them in the past, and they can be extremely useful - particularly if you want to get to/from Linate in rush hour (the rather ugly Lambrate station avoids much of the congestion...)