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As a melee DPS, the choppa has very little trouble handling himself in solo PvE. You can easily take on monsters at or above your rank in rapid succession. You won't really be hurt by choosing any particular mastery path for solo PvE, however, the best one is probably PoDS. PoDS is a high rage, high damage spec for taking on one or two monsters at the same time and once you're high enough rank to train the self-healing ability, your downtime will be nill. PoDW is also a viable solo spec since it focuses on AoE. Your downtime will be higher than PoDS but you'll be able to take out several monsters at once.
PoDW really shines in group PvE situations, such as public quests or dungeons because any balanced group will have a healer to make sure you don't die. AoE groups are the fastest way to level in WAR. Basically, the tank grabs aggro of 10-15 NPCs and then one or two AoE specialists cut them to pieces in mere seconds.
The primary thing you need to be concerned about in PvE is stealing aggro from the main tank, which can be easier than you think. This isn't an issue for trash mobs, but you probably don't want to be hit by a hero or lord, especially when your rage level is at berserk and your armour/resists are cut in half. This can be an even bigger problem when a lack of wards come into play (many groups don't require their DPS to have full wards). Even champions can kill you in a single hit.
It was difficult to test the choppa under normal RvR conditions since the majority of people on the test server were playing choppas or slayers. It would be easy to call the choppa overpowered, but that's just because of the number of choppas present during each attack. Six choppas spamming AoE or focus firing a single target will tear anyone to pieces but that won't happen once the career goes live.
That said, I think the choppa will do well in RvR when there are a variety of careers present. They have an ability that lets them sneak through postern doors, so they'll actually have an interesting job to do in keep sieges. A choppa that knows who to target will do very well and having one or more choppas on the assist train will expediate their target's death. I feel they will perform very similar to the marauder.
It isn't all roses though. Choppas lack in survivability. This is the cost of doing business as a DPS, especially with the rage mechanic. They only have medium armor which is easily debuffed by a number of enemy abilities and they're even squishier when berserk. Their natural health isn't overly high either. It's quite easy to play the choppa with a kamikaze style and many people will. It can also be effective to play the choppa with a reserved attitude. After all, the longer you're alive, the more squishies you can kill.