I've seen ads for it, and it looks interesting, but have never checked it out.
Is it free? I mean REALLY free? In a lot of these "free to play" apps, you have to buy stuff if you want to advance very far.
Tier 1 is free and I'm up to level 10, going for 11 (of 12) in tier 1
You use ship XP and credits to upgrade ships down the tree.
You will need doubloons ($$) on rare occasions, such as purchasing slots for additional ships but for the most part you can play for free.
Now, to gain credits faster as well as purchasing premium ships, the premium section is there but that is an option
You have US, Japan, German and Russian ship options
As an update, I now have a tier 2 dd (finding fighting a dd is a bit more difficult than I thought it would be), a tier 3 cruiser that I have upgraded to be a gunship that I like a lot and this weekend I'll pick up a tier 3 BB (the tier 2 US BB the South Carolina sucks but you got to fight it enough to get sufficient ship XP to go up to the Wyoming)
I love fighting the St Louis (tier 3 cruiser) even though it has limitations (no AA capability (in tier 3 often you play up to tier 4 which has carriers), slow and minimal armor) but I up gunned her and put a lot of HE/AP downrange in a hurry. I've used her XP to get to the BB column and probably will get a US DD eventually. The tier 4 US cruiser to me is a step down but eventually I'll get to the tier 6 New Orleans, which is a kick ass ship.
