Zone 9 Tropicals is one of the best tropical plant nurseries around and Bob is great to deal with. Georgia Vines sells mainly on Ebay and has decent plants. I couldn't tell you about the other two. A great resource for checking out online nurseries is Garden Watchdog which is part of Dave's Garden.
Karyn

Here is a link that might be useful: Garden Watchdog

Thanks, karyn. That is a great site.

From the Garden Watchdog:

Gurney's: 176 positives, 70 neutrals, 180 negatives.

Johnny's Seeds: 191 positives, 11 neutrals, 14 negatives.

Henry Fields: 62 positives, 25 neutrals, 71 negatives.

Zone 9 Tropicals: 53 positives, no neutrals, no negatives.

Seedrack.com (whatcom Seed Company): 33 positives, 5 neutrals, 28 negatives.

Rainforest Seed Company: 1 positive, 1 neutral, 1 negative.

Georgia Vines: 26 positives, no neutrals, no negatives.

And since I mentioned them:

Grassy Knoll Exotics: 12 positives, 1 neutral, 2 negatives.

(I wouldn't exactly call Grassy Knoll's ratings mixed--they are almost all positive, but I would add that I have received great plants from them and Elizabeth is extremely helpful. I'm pretty sure they have the best selection around).

So overall, it's pretty clear cut as to which companies people have had good vs. bad experiences with. Do people agree with these ratings?

A related question: what are the best sources for Passiflora seeds? That is, those with fresh seeds that germinate, are correctly identified, reasonably priced and that have a good selection.