Part One: Mac and PC
I'm a mac user. I've always been a mac user. I will always be, for the foreseeable future, a mac user. This means that anything that doesn't work with some ease on the mac is something I'm not going to use. Especially not a roleplaying tool—I've deleted my bootcamp partition, I'm not going to rebuild it. Installing a wine wrapper counts as "some ease" for me at this point due to my quest for usable windows software that I can run on my mac. So props to FG for not only having a product you can put in a wine wrapper, but for hosting a thread on their forum that tells you how to do it if you're not familiar with things like winebottler.
Part Two: The Rules
Fantasy Grounds comes with some rules installed. These are, predictably, 3.5E D&D, 4E D&D, the Fantasy Grounds "core RPG" rules (not rules at all, merely a framework), Pathfinder, and Numenara. These were DEFINITELY not rulesets that my group was going to use. We play AD&D 2e exclusively when it comes to D&D games, and that's what we play most. For me to get any use out of this thing I was going to have to go on a quest.
It wasn't very long or hard to find pointers to other rulesets. The FG community has been developing them on their own time. And what's that? An AD&D ruleset ready for use right there. Not to mention Mongtrav, WHFRP, Ars Magica, and a bunch of other stuff? Alright, seems like I was worried for no real reason. Of course, once we all installed the AD&D rules and looked at them we realized that A) they weren't complete and B) they didn't integrate well with the updated version of Fantasy Grounds. You couldn't resize image windows or the chatbox. There were no X buttons to close character sheets, necessitating a right-click close operation. Yuck. Before we could use this thing, we had to update it.
Alex and I went on the monumental task of parsing the .xml rule files and figuring out how things worked. Over the course of a 20-30 man-hour week we managed to hack together a frankenstein ruleset out of the old AD&D 2e rules and the new coreRPG rules provided with the updated version of FG. It was not easy. It's still not done. It's usable though, if you click that link you'll find yourself at my dropbox address for it.
This was a massive barrier to entry, but luckily it only had to be done once.
Part Three: The Features
I'm gonna break this down real easy to start. Things I like: linking stuff together, importing images, battlemaps, party roster sheet, calendar, /ooc voice so I don't register all that greentext the players are constantly spouting, auto-calculated AC hit when making attack rolls. Things I don't like: shuffling through pages to find NPCs to spoof or stat, shuffling through pages for ANY REASON, rewriting vast swathes of rules, having broken the tables in our ruleset due to the way the AD&D sheet interacts with the chat box, my players complaining about the difficulty of learning and adapting.
Linking. You can link stuff together. Seems simple. Hyperlinked notes are a wonder. Unfortunately, they are much harder to navigate in practice. I'll stick to keeping my notes on paper.
Images. Best part about this system is the fast way to share images. I have maps of the great city of Miles, its environs, etc. all shared. I play online exclusively these days. This is the only way to ensure everyone's getting the information.
Battlemaps. I'm against lots of drawings and character portraits that are generic and all that. However, battlemaps can be handy in certain situations. Served us well in the manticore fight.
Party Sheet. Awesome way for tracking communal items and treasure. Hands down good feature.
Calendar. Boom, no more linking to a google spreadsheet I custom tarted up. Great feature. Had to program the calendar myself, struggled with zipping the folder that contained it over and over (har har, just zip the FILES, idiot) but GREAT feature. Needs to have the ability to keep track of multiple years, though.
Voice Modes. Great. Love it. Certain players hate it. Hate holding the extra button, typing /o, /a, whatever. Bad times for me, but at least it cuts out all the OOC chatter that clogs up the channel in IRC.
AC-hit. Thought it would be amazing. In practice it saves not that much time.
Shuffling through stuff. Screen real-estate issue and tactile nightmare. I prefer my actual notes.
Spoofing NPCs. I thought I'd hate this, but I've become proficient at it in three sessions. I'm cool with it now.
Writing the Rules. MAJOR bummer. We did it so you don't have to.
Player Complaints. A few. They will be given voice at the end.
Part Four: To buy or not to buy?
Do you have a large amount of disposable cash burning a hole in your pocket? Buy the Ultimate Edition so your friends don't have to pay anything. Otherwise, weigh your options. If you play PRIMARILY or COMPLETELY online, this is a way to bring that "table feel" back to the game. It allows for easy file management and image sharing, which is paramount in my experience.
HOWEVER, the price is very steep. Do all your friends have money to spare? Can you just play on IRC instead? Don't worry about it. This is a gaming luxury item. It's great to have, even with the headaches (unless your rules don't exist on it, in which case you have to be a masochist and a programmer to get them working, and aren't those two the same thing?) it adds a lot to the game.
Bottom Line: You got the dough and it has your rules, buy it. You don't, stick to IRC. If you want to ask me about specific features, email me at email@example.com and I'd be happy to go over them further and help you decide.
Player rebuttals and additions coming soon.