Slap Straps are made out of nylon. It's a fairly high-stretch material, especially when wet. Which means that it's great for things like paracord and climbing rope (where you don't want the guy on it to come to an inelastic stop at the end, breaking bones), but not so great for hammocks.
For hammocks, the current wisdom is that polyester webbing makes the best straps due to a combination of lack of stretch and strength-to-weight ratio.
With stretchy straps, your load will be supported at first, but overnight you're likely to wind up being a lot closer to the ground (possibly catching the hammock on something sharp and dumping you unceremoniously at four in the morning).
There are a number of alternatives. If you're looking for ease-of-use, cinch buckles and polyester straps seem to be the standard. Both Arrowhead Equipment and Warbonnet sell them, though you'll probably want a climbing-rated carabiner or a set of Dutch 'Biners or Dutch Clips to make putting the strap around the tree easy-peasy.
If you're looking for lack of bulk and the most weight savings you can get, the common wisdom is that it's hard to beat whoopie slings, which can be purchased from whoopieslings.com, Arrowhead Equipment, Jacks R Better, and many others. There's even a DIY guide on the forums here, should you want to try your hand at making some. Apparently, they're really easy to do.
Personally, I use a set of JRB Tri-Glides, available from Jacks R Better; however, I think I'm going to be switching over to a cinch buckle system on my next hammock for ease-of-use after seeing a pair of them in use this past weekend. It's only got one more piece of hardware, and it seems as if it would be very difficult to break, so...yeah...
Hope all of this helps.
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