Bought With Fire and Sword on a whim yesterday and have spent a considerable amount of time playing today.
It's a pretty different experience from Warband. Muskets give the game a greater strategic element and reduce the importance of armor for the player, which is nice, especially if you prefer the style of the lighter armor over the heavier stuff. The faction-restricted troops cuts down on Warband's issue of sorts where the optimal way to play was to amass a horrifying horde of troops from all over Calradia regardless of whether or not you were making a pan-Calradian empire; similarly, the apparent removal of defection was a smart move, because it means you don't have to deal with nonsense like ending up with Emir Haeda's fearsome army of Sarranid Vikings. The majority of troops being very cheap is also nice; it lets you deal with large bandit groups in the early game while still not really being comparable to national armies, and cheap mercenaries make nice filler for professional armies that have taken losses and haven't had time to replace them yet.
Muskets are really, really fun. They're really satisfying to use; something about the kick they pack makes landing a shot feel nicer than using an arrow or bolt. The long reload time is actually better for them than crossbows, in my opinion, because it's much more clear-cut when you'll need to cancel your reload and draw your blade. I hear a lot of complaints about getting sniped out of the blue making them annoying to deal with, but honestly, basically the same thing happens in Warband in different forms (e.g. Rhodok marksmen, couched lances coming from nowhere,) and it won't happen often if you make good tactical decisions (like staying moving and using your men as cover. Plus, they make random bandit encounters in the streets at night much more engaging; they go from cutting down some pathetically easy goons to interesting tactical encounters where you have to use buildings as cover.
The setting is also pretty cool. It's not one that gets explored often today, which I like a lot. The aesthetic is very interesting IMO because it portrays a lot of different lifestyles and cultures; it's jarring in a good way to go from meeting a Swedish lord in his fine Western clothes and opulent hall to battling a cossack in his rough, exotic adventurer's wear in the wild, open steppe. (also bonus points for including Janissaries as a Crimean troop)
There's a few things I don't like and/or miss; buying businesses was a good feature in Warband, the renown requirement for mercenary work was the source of much confusion and frustration for a while, the quest where guild masters try to get you to stop a war is offered way too much for the ridiculous requirements all the solutions have, and it's rather difficult to make money without gunpowder exploitation. But I feel that these pale in comparison to the fun, new take on Mount & Blade that With Fire and Sword gives.