I posted this on another thread yesterday...
For those of you who have not yet discovered the joys and convenience of mail order gardening, these snaps show a parcel from Crocus that arrived last week. I ordered on the Wednesday, and the parcel arrived on the Friday...
Pretty much the same conclusions as Verdun has already described.
You can get much better choice than is avaiable in any garden centre. The order above included some Hemerocallis. My local GC stocks 3 varieities at the most; Crocus have around 15 varieties. A nursery specialising in that plant alone may have upto a 100 to choose from.
I've bought loads of plants, of all kinds, by mail order.
Another big advantage is that you don't waste money. If I take a trip to a garden centre, then I'm inclined to come back with a load of stuff that I didn't really need. Buying on-line makes purchasing a more considered activity.