Tauto: Is it as good as the reviews make out or are they slightly overinflated?
I have only played Dark Arisen, so can only provide my thoughts on that. I played through the first time, tried to do everything possible including collecting the 100 tokens. This was a bit over the top, so if your a completionist bear in mind it will take you a long time. Still on the first run through, the game was very entertaining. The fighting in it, jumping all over beasties was really cool, combat was a joy. Experienced no real life changing bugs or anything it all worked, and was pretty smooth. The ability to change out your character into fighter/mage/archer at any point (later on in game) was pretty cool, and necessary to get all the tokens. Storyline was average, can't remember much about it though it did keep things moving. End of the day the big seller is the fast slick combat, so if you like that you will love this. I found the expansion dungeon very hard, even with a high level character.
I re-installed some months back for a play through and only really got to the Ogre. Really felt the "I have done this all before" quite strongly so gave up. So I would say here, replayability is limited. The only real choice you have characterwise at the start is rogue, warrior, mage, so your options are limited there. Later on you can change this across the three, and dual, but even so, character building is a bit limited.
In terms of is it worth £8, I would say yes. The first run through was hours, it was very long, and has lots of content including the new super hard dungeon. It was very entertaining and runs well, but does lack the depth for replayability. Would I buy it £8, heck yes!