My 10 year old wanted this for ages, but I refused to buy it due to the poor reviews. However, it was on offer for just under £18.00 here, and she bought it herself.
She absolutley loves the game, and has spent hours playing with it. The problem I can see, is that if you don't manage to keep the character's needs just right, you end up in the hospital and need to buy medicine, which if you have no credits for, can only be bought by buying the additional cartridges.
The cartridges expand the play value, and you can get the 4 pack for £4.49 from Amazon. The problem is that there are 12 packs and you are sent them at random, so the only way you know what you'll get, is when it turns up, and it may be one you already have!
Argos do the two pack for £5, and you have the option for reserving and viewing before paying, which is probably the only way you'll get additional cartridges of different numbers.