Well, that's a good question.
Personally, I have the stock fly, and have been through some good storms. I would be lying if I said I would be happy with the coverage, but it mostly has worked out.
I just bought an underquilt cover from JacksrBetter, made from a
Driducks poncho that I think will be enough for the side rain where I'm a bit low on the fly.
One thing - if you tie up the fly correctly, and aren't pushing the weight limit of the hammock (like me), you can get a pretty nice coverage - see
Derek's page One more item - you can always tie the fly separately from the suspension, and keep it where you want it. See Derek's video of a
continuous loop.