Hands on with Glu's freemium twin stick iPhone shooter Gun Bros.

Mobile publisher Glu Mobile raised $13.5 million during the summer, such is its belief that free to play games are the future.

It's got a bunch of such titles to release before the end of 2010, and the first is topdown shooter Gun Bros.

It's a fairly simple game to play as you take control of one of the said brothers and then move around levels in the usual twin stick manner, trying to wipe out as many waves of attackers as you can, before you, in turn, are wiped out.

In total, there are hundreds of enemy waves spread across ten planets, and you can always choose where you want to start.

During this process, you collect xplodium, which you convert into coins at the end of each mission, as well as gaining experience. Combining the two, you level up and unlock the ability to buy new items - in total 30 guns and 50 armour.

Helping you along the way is your Smartilley - a big red panic button - that enables you use the power ups and health packs you've bought with your coins.

Of course, the main focus of this activity is to get you to pay money to Glu in order to be able to buy better equipment, although these are level locked, so you can't just pay your way directly to success.

Gun Bros. also supports Apple's Game Center social platform, and does so in a neat way.

Because each game sees your Gun Brother fighting in a duo with a computer controlled friend, if any of your Game Center friends have Gun Bros., you can select their character to play in your game.

Now, this isn't real-time co-op, but you both get additional coins, experience and bonuses when you share characters. And, of course, sharing characters is a great way to get people to customise them, which feeds back into the whole freemium model.

In terms of how the freemium stuff works, it's a fairly standard conversion between the in-game currency - which consists of coins and war bucks - and real money.

If you can't be bothered to wait around earning coins, you can buy 5,000 coins for 99c or 59p, or 53,000 coins for $9.99 or £5.99.

War bucks cost 99c or 59p for five, up to $9.99 or £5.99 for 65. There are various options inbetween these limits.

You can also pay to transfer between coins and war bucks at a rate of 1 buck to 500 coins, or 5 bucks to 3,000 coins.

Another interesting aspect of the game is the way you convert your xplodium into coins.

You can do this immediately at a 1:1 exchange rate, or alternatively, you can buy better refineries that take longer but give you more coins. For example, you can wait 24 hours to yield five times the standard xplodium-coin transfer.

However these options are expensive to buy - the five times one costs 130 war bucks (or $20 in IAP terms); but again it does encourages you to spend real money to improve your in-game experience.

Still, Gun Bros. is an interesting approach to the freemium model on iOS, that sits somewhere between the casual social nature of FarmVille or We Rule on one extreme, and the hardcore real-time nature of something like Eliminate.

Gun Bros. is out now for iPhone and iPod touch, and is free to play.