Not sure whether I've ever talked about this before but along with Bloodbowl and Space Hulk we tend to use this as a regular break from 40K. There's four of us who regularly meet up for a running campaign. I only joined the first campaign half way through but I learnt a lot from it. So far in the second campaign I've won 10 out of 11 fights with my Cawdor. It's a really great alternative to 40K that's easy to get into and I'd recommend it to anyone. Some GW stores and FLGS have campaigns running but it often get's overlooked by people because Bloodbowl is much more popular (more on that soon).
For those of you not familiar with the game, it follows the 2nd/3rd edition 40K rules but instead of an entire army you only have a gang of fighters. The action takes places in the hive cities of some planet (maybe Terra) as gangs fight for control of the underhive. The main advantage of taking up Necromunda is that the rulebook is online for free so the only thing you need to play it is 10-15 models. There's tons of them on eBay so you can pick up a gang and a few spare models (more on this in a bit) for around £30. Although I haven't managed to paint them fully yet, it's pretty quick to get your gang ready for action.
As you progress through the campaign your gang takes on it's own personality. All of the gangs start off pretty similar but that soon changes after a couple of fights. All of the gangs have some combination of a leader, heavy weapons, gangers and fresh faced (but not for long) juves. Juves are pretty poor to start off with but they advance quicker than any other class. After each fight your fighters gain experience points regardless of whether they win or lose. It's still better to win though as you'll gain more experience and progress quicker. There's normally bonuses for being the leader of a winning gang and wounding hits give more XP too.
In contrast to 40K it's unlikely you're gang members will actually die (1/6 chance if they're taken "out of action"). This means that your gang steadily improves in both it's stats and special skills. As your gangers level up they randomly gain either stats increase or skills based on the particular house they come from. My gang is from House Cawdor which means they're ferocious combat specialists. Just like in 40K you've got to survive being shot at if you want to make it into combat so it's important to use cover wisely. Those of you who remember the old editions of 40K will know that cover confers a to hit modifier instead of a cover save. Personally I prefer this system to the current 40K one as it's a bit more realistic. For example, if only your toe is in cover you don't get a 4+ save but instead a -1 to hit modifier. Having said that the rules are far from perfect and there's no living rulebook (that I'm aware of) like there is for bloodbowl. This leads to house rules on things that are ambiguous. It's a shame GW don't give it more attention or else a complete revamp but since it isn't as profitable as 40k you can see why.
The real joy of Necromunda comes from the necessary conversion work. Not only do your fighters improve their skills but they also have access to a market for rare items such as power weapons, armour and grenades. It's fun to raid the bits box to represent these new weapons on your models. It's handy to have a few extra figures though as in subsequent campaigns you obviously revert back to basic equipment at the start.
Whilst it's easy to replace a knife with a power sword it's more difficult to represent other upgrades. The original models are all metal so that can be frustrating. However, I'm pretty pleased with my flamer conversion. It took a bit of carving to get rid of his autogun but it was definitely worth it.
The other advantage is that games are usually short at anything from 30 mins to 2 hours for a real meatgrinder. In fact I enjoy the after game levelling up almost as much as the actual fight.
Anyway, if you haven't ever tried Necromunda I heartily recommend you give it a go.