Originally posted by olliedell I did have a look but that's pretty expensive, I was thinking there'd be a pentax lens close in price to the Tamron (£130) that you could recommend.
You wanted, and I quote: "obviously I want a quality lens but I also want it to be as cheap as possible". I believe the 55-300mm fits that bill near-perfect but I cannot look into your wallet obviously.
If that still is out of budget, a couple of steps down would be the Pentax 50-200mm which I believe can be had for prices comparable to the Tamron you were looking at. I honestly believe this particular Tamron is one to be passed up due to excessive CA, slow focussing and below-average build quality. The 50-200 is quite a deal softer than the 55-300mm but certainly suits the "budget" requirement and seems to be better than the Tamron.
Some shopping around might even find you the FA-J 75-300mm which is passable (barely) at least up to 200-250mm, but still trumps the Tamron and at an even lower price.
Now, I've been in these discussions before and if you have already set your mind on the Tamron and want confirmation you made the right choice, you will not get it from me. Likewise if you already bought it and all you want is reassurance that it's a great lens in order to justify your purchase - again, I'm not going to provide that.