I usually get good service from them, but you have no way of knowing if an items in stock, or how long it'll take to get it if its out of stock.
Avia is a case in point. I cancelled my order from them last week, after waiting 3 weeks for it, with no info other than a 'you still have this on back order' email from them.
I've had this a couple of times previously, but generally they are OK, especially if you are patient.