Who doesn't want to be a space ninja?
The game is great for free to play and most fun is had when you work with friends. It can become a problem later on if you do play with randoms as you may need to collect loot whereas they may want to race to the finish. You then miss out on the bonus creds for finishing the level. Loot is necessary to build new weapons, warframes and sentinels when you have acquired the necessary blueprint. This is where the game becomes a little grindy. You sometimes have to repeat the same boss and same planets over and over in order to get the materials or blueprints that you want. (Update: Blueprints have been made easier to collect) You can however purchase them with real money in the shop and some items such as new skin colours can only be purchased this way.
The levels are very samey and the map is randomly generated; you can sometimes run through an identical room two or three times in one level and that doesn't include the required backtracking.There is also a limited number of planets at this time and you can only level each warframe up to 30 which doesn't take very long to do. Perhaps this will be addressed at a later point as the game doesn't provide much incentive to keep you playing. The game doesn't scale down if you want to play solo, so it can be a refreshing change of pace and is great for someone that wants a serious challenge.
Warframe can be very fun and I'd certainly recommend it to anybody who wants to hack through large waves of enemies and feel awesome doing it. It does become quite repetitive but it is definitely worth a go since it's free to play.