Any more of you out there played this? I only started playing just recently but myself, Ava & Shin have got together for some of the games (them two more than me since they've been playing longer and i've been logging off just minutes after Shin has been on this week so haven't had a chance to play with him this week unfortunately) but it really is fantastic fun and if you haven't already i'd recommend trying it.
The MP is great that you can just drop in and out of every now and then and so doesn't require lots of time, whilst there are things to aim for for those that want to dedicate lots of time to it as well. So unlike a game like GTA4's MP (imo), which is fun but the only actual goodies you get for getting a higher level is new clothing, there's lots of incentives with WaW to want to keep playing.
It works similar to an MMO leveling system (starts with level 1 - 65) and the more enemies you kill, the more XP you get. You also have challenges there to try and complete as well, these can be things like "Achieve 25 headshots with X weapon" and by completing those you receive an XP bonus and in some cases mods for the weapon you completed said challenge with (eg. Special sights, suppressor, Bipod etc). At certainly levels you'll unlock better weapons as well so there's always those to aim for too. Whilst some of these are really good, and a noticeable improvement over the ones you start with they don't actually make the higher level people overpowered as it were, the lower ones can be just as effective in the hands of a skilled person.
There's also a feature to "create a class" so you can have a nice selection that you can easily pick from at the beginning of a game. Say you're playing on a map where there are tanks involved, you may want to create a class and name it "anti-tank" like i did, and select some effective weapons for that like the bazooka for instance..
You also have perks that unlock at certain levels too so those are another thing to aim for, some of those can give you things like bouncing betties, extra special grenades (frag, poison etc), deeper bullet penetration, drop a grenade when you die, extra health etc. Good thing about those is that you can only pick one from each section and there's a good few that i can see as being quite beneficial and so not everyone uses the same.
When you reach level 65 you have the option to enter prestige mode too. Doing this resets your level back to 1 (which will remove everything you've unlocked as well) but gives you a special prestige badge next to your name. Prestige has 10 levels (and 10 badges), so to get to prestige 2 you'd have to level up from 1-65 again just like you did to get prestige 1, and so on. Doing these opens up extra "create a class" but other than those i'm not sure if there's any other rewards at the end other than the 2 achievements (which stupidly give 0 points) and the 10 badges. Either way still lots of incentives overall for the people that want some type of progression.
Anyway, sorry to drone on a bit there lol. Thought i'd explain a bit more about the MP feature since it really is excellent with all the options available and there's more maps on the way too. The 3 of us have been playing team deathmatch and don't use mics, but still, lots of fun. Those games consist of 12 people as well so with another few people joining in we could pretty much fill an entire team with SSX. Soo, if there's any of you online in the future and see Ava (Avapops), Shin (SSXShingen) or myself (Rigby85) partying, feel free to join us, the more the merrier.

