This is a nice little expansion that gives you some nice new Gather cards you can use in any game, and also an alternate set of Craft cards for a faster game. The new Craft deck is easier to build, so you'll need to invest less time and thought into building your dice pool, but you'll be crafting items faster (and more often more than one per turn).
This is a really rare type of expansion - one that speeds up and simplifies the game. However, I think it is a good idea, because as much as I love KF, it can take a while with 4 players. With this new deck, you can reign the game down into the superfiller category and perhaps play with more casual gamers who want more dice rollin', less Gather Phase strategizin'.
At first I was afraid this would complicate things too much, but it actually integrates very well and adds a nice additional layer of choices. It will prolong the game a bit, but the game is one I love, so that's fine with me.
I was very surprised how much more interesting the game becomes with the addition of these new cards. The expansion definitely creates a host of new potential strategies, and makes me wonder how I ever felt the game was complete before. this expansion has definitely slowed things down for me, and I find myself changing strategies more mid-stream. However, I do enjoy the added depth and options.
I am not a fan of the takeover mechanisms, however. Too much bookkeeping for too little gain! From an interaction standpoint, I'd rather see ways to stop your opponents from taking particular phases.
I was skeptical about this expansion for quite a while, but after 50+ plays with my wife I finally decided to bite it off. Adding to RftG is not a no-brainer for me, despite how much we play the game. We don't play with takeovers. That whole thing seems like a mess to me - way too much bookkeeping for too little effect.
I was worried that prestige might also be a bridge too far, but after playing with BoW I do not find that to be the case. BoW is very much meant for experienced players, but the added rules burden is not too bad. I mainly like 3 things about the expansion:
1) The prestige mechanism gives you another interesting victory strategy to peruse. I don't find it overpowering as some have commented here; there are many other strategies that can also yield at least a point and a card every round!
2) I like the prestige super-ability one-shot. It is frustrating to get stuck at the midgame or even early on, and this feature gets you around that at a cost.
3) Green strategy gets some help. I felt that the green engine was relatively under-resourced before.
I could have lived without this expansion. I need to remind myself how prestige works when I pick up the game after a while, and I still need to look up some of the trickier BoW cards. Search also slows down the game quite a bit. However, it is hard for me to go back now, because BoW does introduce some interesting strategic options.
UPDATE Sep2012: Alright, dangnabit, I need to admit that this expansion completes the game. It definitely slows things down, but it also easily doubles the number of strategies possible. It's really well positioned at the end of the RftG line. When you're ready, get it, and you will come to appreciate it greatly. I take back what I said about being happy to live without this expansion - there's no going back for me now.
Solid adventure card expansion. Challenges are a bit harder than average, with some interesting effects. Adds some new Events that could be potential huge pains in the butt, but that makes them meaningful. I like the encounter that allows you to attempt to loot a dragonlord.
The best part for me are the new Blue challenges that can only be damaged in a single combat phase. I like when you occasionally get something like this, and realize you need to run away for now! Also includes some Encounters that basically give you an extra hero to use just for stalking your opponents. I don't think PvP is a strong suit of Runebound (we usually avoid it), so these didn't do much for me. The other monsters are decent.