Are Tenergy LifePo4 batteries any good?

An 800 watt skateboard is big enough to need a good lifepo4, not just a few cheap cells in a small amp hour size. To have the pack last, you should use something that could drive a similar wattage bike. Usually this means about 10 ah of cells unless you are using more expensive stuff. If it is for short rides, then you might want to look at higher discharge rate lipoly, found in the hobby stores for helicopters and such. That way you can still have a small pack, lotsa power, and less money going out. The downside is having to charge carefully, in a place that won't burn down. They have fireproof bags for that though.

Edit, Just back from the site, actually those aren't bad looking cells. I was thinking you were looking at cells in the camera size. You'd need 12 of em, a bms, and a charger that goes to the right voltage. Don't even think of no bms. It might be easier to just order the same thing, assembled, with a charger and bms from high tech bikes, or pingbattery. At 8 pounds, you could easily carry it in a backpack.