Hm, I am
sure I replied to this last night!
Anyway:
The plan seems reasonable.
Prepaid 'minutes' are a bit more expensive than those under a subscription plan, but not prohibitively so (say, $0.10-ish a minute within a given country and network, perhaps 20%-50% more between networks). However, the SIM card itself is not always free (and some operators charge up to $30 or so for it).
Virtually all subscription plans and most prepaid schemes come with international roaming, but you don't want to use it more than necessary: for example, roaming between Croatia and Austria (almost neighbors) cost me more than $1.50 a minute, on a subscription plan! So, if you only plan to send a bunch of SMS messages, say 'Hello' once or twice and have a phone for emergency, don't bother with more than original pre-paid SIM. Otherwise, buy a new one in each country you visit.
Another point: it is not always possible to 'fill up' your SIM from country A while in country B. Obviously, choosing an operator active in both countries might help. See, for example. T-Mobile, Vodafone, Orange...