I buy pretty much everything online, unless it's a high ticket item (eg I bought my plasma TV at a local store rather than saving a bunch on buying online because I wanted someone to install it and take responsibility for any problems there might be).
Things seem to have got worse over the last year or so. "In stock" used to mean "In stock" not "As soon as we've taken your order we;ll change the order to "Temporarily out of stock" and then leave you waiting for weeks. Amazon are the worst, especially with pre-orders.
The biggest problem I have with online ordering is the delivery companies who are ALL rubbish. You wait 2 weeks for an item and the company claims they "attempted" delivery 4 times and left a card when you'd stayed in all day and they hadn't - the driver just figured he'd fake it. You rearrange delivery for a specific day and they don't show up, with no explanation ever given and the online ticketing just mysteriously showing no update. etc etc etc. It all just adds too much stress and way too much time off (or staying in at home) so much so that I'm actually moving to the alternative model of picking up stuff myself from bricks and mortar stores where I can have some control over WHEN I get the wretched goods, even if it costs more money.