Based on our experience and challenge that is on offer with Onslaught mode, we'd most certainly be happy to recommend Onslaught mode for Bad Company 2 veterans, as there's enough replay value to warrant the price tag, and with new maps on the horizon Onslaught will undoubtedly be a mode to revisit in the future.
Onslaught offers basically the same multiplayer experience you've been playing for months. If you don't like playing against human opponents and if you're looking for a co-op mode against bots, this DLC gives you 4 maps to beat. It should have been free for VIP code owners, though.
Bad Company 2 remains a superb shooter (if still rather borked by that last patch), but having gorged on it for three months we need something more interesting than second-hand spaghetti bolognese if we're expected to pick up the tab.