I started with the original game well before it was finished, back when you paid during the development of the game and you didn't have to pay more when the game was finished. I was invited to be a beta tester for Warband, and I did that for a while. However, I have never liked multiplayer games, and I don't currently play any of those now. I dropped out of the beta testing after a few weeks, because all there was to do was test multiplayer battles.

I still enjoy playing the original game. I start a new game every year to 18 months and play until I get bored with it (usually for a couple of months). I have the game tweaked exactly as I want it to be. I keep an older save game with a mid to high level character, just outside a city with a tournament, and I play that fairly often when I'm bored, or if I've had a bad day and I just want to kill some people, and I'll do the tournament a few times and then exit the game.

I only enjoy playing a total cavalry army. I will only take on lower level recruits that will train up to somebody with a horse. My armies usually mainly consist of Swadian Knights, Kherghit horseback archers and lancers, and a few Vaegir Knights and Sword Sisters.

GoG is running a very good special now with all the games for a really good price. If I'm quite happy with the original game and I have no interest in multiplayer, is there a reason to buy Warband, The Expansions, and With Fire & Sword and and play them totally as single player, and is it possible to tweak the game as much as the original game, considering that these are primarily made for multiplayer?

I read through several pages of posts, and I'm sure this has been asked before, but I couldn't find a good thread or answer. So, because I was only willing to read for about an hour before asking a question, I have posted this video to YouTube as a mea culpa and a bonus for anybody who might help me with my decision.

https://www.youtube.com/watch?v=9wTIuLzo0hs

Warband is an improvement over the original M&B in every aspect even in its singleplayer mode. Plus it allows you to download and play Floris, which is pretty much as much of improvement over Warband as Warband is over the original.

Don't buy Napoleonic Wars if you're not interested in multiplayer, because it's multiplayer only (unless it turns out for you cheaper to buy it, too).

Viking Conquest does have a single-player campaign, though, so you might be interested in that. I do have this expansion, but haven't played it much. What I know is that the map is HUGE in this one and it also has a storyline (but you can play "sandbox mode" like in original M&B, too). You can also start the game as a vassal or even king, if so you choose. I'm also very fond of Viking Conquest's soundtrack. :P

With Fire & Sword also does have a single-player campaign. If you like the setting and historical period, I think it's worth giving a shot.

So to sum it up:
-Warband - you can't go wrong buying it
-With Fire & Sword - if you like the setting, buy it - no real innovations other than that
-Viking Conquest - many innovations you might or might not like
-Napoleonic Wars - multiplayer only, so buy only if it makes the bundle cheaper

That is pretty amusing. :D
I honestly never even remember to jump in combat, especially when mounted. I might have to try this now!

As to your question -- I've also only played the original, and I feel it works quite well as a single-player experience (if you like sandboxes, anyway). Obviously, this makes me less qualified to advise you. I will say that I've heard that there are tons of good mods for Warband (by this point, probably more than for o.g. M&B), so if that's part of what you meant by being able to tweak the game, then I'd say it'd be a step up.
I'll also second Taro94's suggestion to skip Warband's Napoleonic Wars DLC, as it is apparently only intended for multiplayer.

Thanks for the answers! It was a really good price for the bundle, but after thinking about it, I decided not to buy it. As I said before, I already had the original game, and it looked like I wouldn't be playing 2 of the newer games, so that made the bundle less desirable. I've decided to just buy Warband and go from there in the future.

I have had the original for a good while now. I just installed warband and have played for about 300 days (game time). I play single b/c I have slow DSL. I am a lancer w/ a big axe guy. The new game's aiming system is driving me crazy! I hope as I advance in skill sets, things will get better. I guess I could sell my horse and become a foot soldier. Maybe there is a mod out there that gives me a lance that doesn't bob up and down as I gallop toward the bad guys.