They're great on pre-orders (always get mine before the official release date) but struggle with older titles, and seem to have got a lot worse over the last 12 months. I've currently got several very recent titles which have been "Temporarily out of stock" for more than a couple of weeks now.
They are subject to the vagaries of the British postal service - delivery of "first class", if I can believe the despatch emails sendit.com send out, vary from "next day" to "a week" with the latter being far more likely if several titles are packaged together
They are particularly bad with HD-DVD and Blu-Ray titles - I think this is just a "lack of demand" and "difficult supply" situation that most other retailers are also facing. They clearly don't keep much stock of these.
Communications used to be fantastic with an automated email system that told you how long it would be before they got back to you. These days there seems to be just one person dealing with the email and he's out on Mondays, but generally they're very good at getting back to you eventually. Over the last 10 years I've had a few issues with non-arrival or wanting to return a title (eg a "This Film is Not Yet Rated" which was widescreen format but presented letterboxed in a 4:3 format) and they've never quibbled and even given a freepost address for return.
I use them for all my UK pre-orders, despite feeling that they were so much better (bigger?) when they were blackstar.