They're on sale on the 360 this week and I'm sort of tempted by them, can anyone give me a rundown of what sort of stuff are in them & if they're any good? I'm most likely to solo them if that makes a difference

Zombie Island - I'd say it's worth it. It's a mini campaign which i'd say is about a quarter of the length of the main game. Only thing I recall it adding is more missions and stuff like that.
Mad Moxxi - Meh. It's an arena thing, nothing more and nothing less. I never got to finish it so I don't know if there's anything more to it.
General Knoxx Armory - Got it, but haven't played it.

The armory also increases the lvl cap. Dont know how high.

That's pretty much what I would say. Mad Moxxi would have been better if it had saved arena progress, but nope. You gotta do the whole damn lot in one sitting (something I will never have time for). The only redeeming quality of the DLC was the bank storage, but even that could have been better.

Zombie Island - a nice addon definitely worth the money. It offers a nice change of scenery, new enemies and fun gameplay.
Mad Moxxi - get it only if you have at least 2 friends to play it with online. It offers a really hardcore challange on playthrough 2+
General Knoxx - for me it ruins a balance of the game by increasing the level cap. The problem is that new stronger enemies appear only in the addon. Therefore, once you level up to new 61 level playing the original campaign is just pointless because it gets to easy. You should skip this one.

I bought Knoxx for $3 at the Steam summer sale and it seems to be the only worthwhile one -- subject to the disclaimers listed above me.

I only got the General Knoxx add on and must say that it's good, it adds up about quite a bit of quests and maps, lvl cap raised to 61, new rare tier of weapons and shields(Pearlescent/cyan color) and there's also a "raid boss" at lvl61 where you need a decently equiped group to kill him even on plathrough 1, but as it was mentioned it does make the original game pretty useless