As far as PLAYING 4e on Fantasy Grounds goes, JPG's 4e ruleset is amazing and completely superior to the built-in 3.5 ruleset. It's at version 1.5.1 now and has basically covered in detail every nuance of 4e. I would actually use FG if I ran offline games, that's how useful this software has become with JPG's ruleset.
As for prep... I know several 4e Fantasy Grounds DMs and we all have very different ideas of preperation. At its core, FG can be used with NO prep, simply as a virtual tabletop. It provides virtual dice, virtual paper, and Skype provides virtual communication, what more do you need?
But it DOES have a lot of utilities that can help a DM/group. The most time-consuming option is to do a complete conversion of the adventure to FG. When you're good at it, this takes about three to four hours. It can take longer when you're not. Mind you, this isn't prep for the game, it's just converstion.
I tend to do something in between. I use Tenian's parser, which converts text files into FG-usable game elements. For example, those NPC sheets you see around the map are generated by his parser. You'd be using the parser to do an adventure conversion, too, it's just a matter of how much text you want to copy and paste from the PDF. Plus, EnWorld's formatting of the NPC blocks does not follow the WotC standard (though it's very close).
All in all, I spend about a half hour using the parser to get the NPCs in. The maps are a breeze, you just copy them out of the PDF and into the FG folder. Last but not least, tokens. This is another thing different DMs do differently. FG comes with a set of letter tokens, which take zero prep time. I like to make nice, custom tokens. They take about five minutes each.
So, in summary... I'd say I do about an hour's worth of work I wouldn't be doing in a normal session, but we get a very streamlined experience out of it and the battlemaps+tokens look great. Most efficient cost/benefit ratio, in my opinion.
Hope that helps.
