I got the campaign content and the My Kong mode (since the Eurogamer review called that the only part of the game worth playing). Haven't played MK yet but I'll just keep it at hand should a spontaneous party arise.
On the matter of the campaign, one issue that reviewers pointed out is the medal grind: Beating a level with a tank for the first time gives you a medal, to get another medal for the level you need to use another tank. There are 36 levels and the last level needs 100 medals to unlock (also the last two tanks need 300 and 500 medals respectively but that makes them sound more like they just exist to give people who 100% the game some reward). The first few hurdles are more problematic, it's something like 20 medals for level 7 so you really won't get much variety when scrounging up those medals. The tanks can be leveled up for some minor benefits which at least gives you another benefit to replaying levels.
The main issue is of course the core gameplay, your tank can't aim sideways so you either drive towards/away from the enemies or you can't shoot or see them. Since maps are fairly small you can't do a constant backpedal like in Serious Sam or Earth Defense Force. Vertical aiming is automatic like in Doom (or EDF when set to simple AKA Japanese default controls) which doesn't help much when you're trying to pick the high explosive red enemies from a crowd.
Oh and you get an AI coop buddy if you play solo which can be moderately useful (especially if it rolls a stunner weapon) to really annoying (because it picks up weapon drops too and may just ninja that superweapon drop and then proceed to completely miss the enemy).
Overall it can be a bit fun for a while but it's really damn shallow, at times it feels much more like luck than skill because you can't do all that much anyway. 10€ for the TTT campaign offers less value per Euro than 40€ for EDF 2017 Portable on the Vita.