I don't think N2 is worth it, imho. I've talked with folks who swear by it, and I've used it, and have seen no difference. :-/
Keep in mind:
1) Pool water is different than hot tub water. The high temperature difference makes for significant sanitationneeds as well as the volume of water to user ratio. What works well in a pool may not work well in a hot tub.
2) Despite what you may have been told, you will need to continue to use Dichlor and still shock. Additionly, any $$ savings you have been promised from reduced chemical use will be offset (and exceeded) by the price of the N2 cartrige.
3) Don't keep old N2 Cartirdges in the spa. After each water changer, or 4 months (which ever comes first) change out the cartirdige. Old units can break open and the ruin the pumps.
That being said, I recomend you wait a bit and get used to your spa, your water and a get a handle on water care. After a few water change outs, try the n2 catrige for yourself and see if it makes any difference to you.
I didn't see any difference, but some folks claim it makes thier water "silker" and "Brighter"