I buy from Amazon.com (several times a year) and Amazon.co.uk (only recently, since GBP fell) quite often and very satisfied with the whole experience.

Normally for most novels, it's cheaper to buy locally (paperbacks), but for some specialty books (Lonely Planet guidebooks, wine books, etc.) it is often very much cheaper to buy from Amazon than from local bookstores even with shipping. Lonely Planet guidebooks cost >RM100 in general, but I can get it from Amazon around RM80 (shipping included), provided I order a few books at a time.

It used to be cheaper for me to order from the US, but now with the weak GBP and strong USD, sometimes it's cheaper to buy from UK. I recently ordered a bunch of Moleskine notebooks from Amazon UK. Much cheaper than buying them here.