I buy all my discs online from the US - HD DVDs from
Movietyme and BluRays from
Amazon.com. I will buy some UK discs - at least for region free HD DVD - but the big stumbling blocks are (for me):
Prices
High Street prices for this discs are absurd for both formats. £24.99 for some gash films is not going to inspire anyone to buy them. You can get them much cheaper online though (
Sendit for example where most HD DVDs hover around £14).
Combis?
On the HD DVD side most UK releases seem to be HD DVD only - I am a big fan of the combi discs - but I suppose a few might see this as an advantage as it allows users to choose whichever they prefer.
Sluggish Releases
Early days yet but by the time a title is released in Europe I normally have the US version already.
Different Cases
This shouldn't be an issue - but it is

Some UK releases have DVD thickness spines. It looks a little gash when on the shelf. This wouldn't be so bad but (unbelieveably) some studios have just deformed the US coverart to fit the wider shaped case! Checkout the spine on Sony's
Ultraviolet BluRay! And they expect us to pay £25 for it!
Regional Coding
Only an issue for some BluRay discs - but a significant one none-the-less. If the new formats fail to become mainstream they
will still survive as a niche market. However they are more likely to survive and prosper in the US than the UK. Ergo it seems unwise at this stage to buy Region B titles.
...I will support some UK releases if there is a benefit. For example the UK release of
Terminator 2 will be a must have for me as it's looking superior to the US release.