Well as I said all ranks are 'free'. The game mechanics just mean that with less XP and less credits earned in each fight it takes much longer to get to where you want to be. XP can only go up but as I said due to repairing and re-arming tanks you can lose credits. Without premium on your account it is more likely that you will lose money. To offset that you can either play a large number of games in lower tier tanks (tiers 5-6) to refill your coffers - it is (effectively) impossible to lose credits at those levels but then you don't earn as many as you
can on good high tier games either. Or, your second option is to buy one of the premium tanks as these print credits so you don't need to play as many credit earning rounds in order to finance your bigger tanks. But, real world money is represented in game by gold and it is impossible to lose gold in game by having a bad round - you can spend gold in a round but that is different to losing it if that makes sense (feel free to tell me it doesn't and I will try and explain!)
So, onto the million dollar question!

I have had 2 months premium (considering a 3rd) so 17-£25 and I have a premium tank that cost about £22. So yea, quite a bit - £40 is about the same as a new release and if you look at things like the graphics then you could question whether it is worth it - I already know what t2d would say

But on the flip-side of that is the fact that you get a patch about every 1.5-2 months and these patches do things like:
- fix bugs/balancing issues (obviously)
- give new tanks - as I said there are currently 4 nations but British tanks are on the horizon and last patch expanded the french line from just mediums/heavies to also include SPGs and tank destroyers. The patch before added a second Russian heavy tank line. The next release adds tier 10 mediums/tank destroyers to all nations and so on.
- 2-3 new maps every patch
- new game modes - 2 new game modes came out with the last patch.
- the graphics got updated fairly recently (before I started) and very soon there will be a new physics engine and then eventually multi-core support probably tied in with further graphics improvements,
And everyone gets this - there is no DLC to pay for. They get their money through the channels that already exist. So there is real and consistent development in the game. You could argue that the new tank releases are an attempt to lock people into keeping premium to get them faster but for people like me I can see that is the case and I'm in no rush!
They are also developing 2 further games,
World of Warplanes (don't really care about this because I suck at flying!) and
World of Battleships (this I most definitely am looking forwards to!) and the credits and parts of the XP system will work across the titles.
I do admit to liking a bit of a grind in my games and this definitely relies on grinding but with 8 tanks to pick from I can mix things up how I want to (I started playing with just enough time to earn myself 2 free tank slots due to tech tree changes in 1 of the patches).
Another thing to point out is that you start a round and these have a time limit of either 10 or 15 minutes but if your tank gets destroyed you can leave that round, go to your garage and jump into 1 of the others so there is not very much sitting around going on