I administer a ~20 user network. How much will roaming profiles slow down the network, and would it be benificial at all to implement them?

Roaming profiles dont slow the network down so much it just an increased time for users to log on to a PC they have never logged on to before.
I think roaming profiles are awesome and should be enabled BUT also edit group policy to stop some folders in a profile to not roam, i.e application data, local settings etc. These just get to big and take up space on your profile location.
As too is it benifical, I would say yes. But if users dont move between PC's and dont (and which users dont) modify profiles then there may be no reason too. But I like them and think yes.

It's not a good idea to use roaming profiles. They are rising day by day, may corrupt after incorrect logout and we can find many other disadvantages.
I can suggest you take a look at desktop authority (scriptlogic.com). This solution can eliminate the use of roaming profiles completely, performing the same functions with redirection of shell and all users (common) folders to home directory on server.