They are most definately cost effective! Make sure you buy quality and don't get caught up in the "show off" nappy hype. Though some particular nappies have great resale value!

Bamboo is a great fabric. It's highly absorbant and compared with cotton you need less of it to hold the same volume of fluid so it's less bulky. It is slower drying though. Hemp is a great fabric too and is in the middle of cotton and bamboo for absorbancy.

Be very wary of ebay sellers. I don't think they are all bad but don't buy a whole stack off one seller. Get one or two and see what the sewing quality is like and how they perform. It's worth paying a tad extra in postage so you don't get caught out buying junk.
I'd give the same advice for any nappy though. Read the forums and google search for reviews by all means but seeing as babies come in different shapes don't buy too many of the one type straight up JIC the don't suit your bubba.