I've had a simmilar experience to others here. Sometimes it works but often it doesn't. Almost all of my bookings are made through the corporate TA, so I don't know whether it works better for bookings through the AC web site.

I also seem to have a lot of problems with other parts of the web site as well. Frequently there are javascript errors, and I can't search for tickets or even search the online timetable. When this happens, the month field shows only 'Jan' and the date field has no values. It's very annoying!

I've recently started using FireFox in addition to IE, and it seems to work in FireFox sometimes when it won't work in IE.