In reply to Rob:
We've done this twice - in 2004 and 2009 - and I'd really recommend it. Slovenia has well-developed tourist facilities on a par, say with Austria or Italy, but surprisingly pricy. We particularly liked Camping Vovk - the climbers campsite for Osp and Misca Pec. Croatia was less developed and much cheaper than Slovenia, but we noticed a lot of improvements to road infrastructure etc in the 5 years between visits, and there may have been much more done since.
In 2009 wild camping in Croatia was strongly discouraged due to land mines - probably the most convincing reason not to wild camp I've been given! However, I don't know the current situation and they may have been cleared by now. The campsites we stayed at were cheap and a little idiosyncratic - in Paklenica we camped at Hotel Alan which allowed us to pitch up on the sea shore and paddle our kayaks straight from our pitch.
We weren't too troubled by the border crossings - yes, you have to show your passports, but it's nothing compared to, say, Canada to US. Probably worth avoiding rush hour etc.
In both countries, everyone was really friendly. Many people speak English, but if you speak German that can also be useful (assuming you don't speak Slovene or Croatian!)
My main concern would be that it will be VERY hot in summer. The Julian Alps section of Slovenia is a little cooler, but better for mountaineering than cragging.
