I use Amazon UK for pretty much all of my video games, as they are one of the only stores professional enough to remove tax I'm exempt from (the specialists all refuse). This also makes them the best value for me.
I agree that their delivery service is inconsistent. Even as a Prime member, they seemingly at random assign either of their two very slow couriers to deliver my stuff... as I'm not on the mainland this becomes a five working day service, not the 1st Class that Prime pays for. I cross my fingers for Royal Mail each time.
Lately they have gotten games to me by the release date, apart from Borderlands 2 PC (two weeks late!) - which they blamed on the supplier not allocating enough stock. I am lucky that I don't care about the major Call of Duty or Battlefield releases, because Amazon seems to screw those up big time. I have, however, had several other releases miss me by a number of days because they chose to assign a courier instead of Royal Mail, and post it the day before.
If I lived on the mainland and had to pay VAT, I would probably use Shopto. Amazon are great if you have to return something/get a refund. Otherwise, you have absolutely no control over which delivery company they get to ship your items, and they really don't "get it" when it comes to release dates. Their release date delivery guarantee is a bucketload of manure half the time.