It's a fantastic game that is also very hard but not impossible.
Prima do a very useful strategy guide for "Ninja Gaiden 2" and left to my own devices I don't think I would have ever made it as far as defeating the Fiend Alexia before the volume of other games I had to play lured me away.
It's a difficult game but not insurmountable.
I am finding the difficulty levels on "Ninja Gaiden 3" are easier.
I played "Ninja Gaiden 2" on Acolyte level but I am playing "Ninja Gaiden 3" on Normal rather than the easier "Hero" difficulty.
NG 3 is definately worth picking up though, especially if you're not going to be someone who's upset that they didn't copy the movesets exactly from "Ninja Gaiden 2".
It would make more sense to get NG2 first seeing as it's much cheaper so you don't have much to lose if you don't like it. NG3 will probably drop in price pretty fast.
Check out the demo for NG2. It's something like an hour long and gives you a good few weapons to mess around with. I don't really know how hard it is but I get the impression some people blow it out of proportion a bit.
If you're casual and don't play games on the hardest difficulty i wouldn't bother. But i enjoyed it thoroughly didn't really find it that much of a challenge accept for the end boss which was a pain, but im used to playing games on hard.
Story wise it makes no difference to the order in which you play, playing 3 first is probably the best bet so you can see 2 for the masterpiece it is compared to the utter tedium of NG3. Playing them in that order will feel like the series has moved on rather
than taken a huge leap backwards.