Sine wave is nice, but probably not necessary for most things one uses from a vehicle 110voutlet. Laptops, chargers just convert to some 5vdc or 18vdc or whatever, so they could care less.
Motors don't particularly like modified sine-wave inverters, but they work and don't really do any damage to them.
I wanted a sine-wave one in the beginning too, but after reading further and talking to some people, I think really not necessary unless your running sensitive medical or some type of diagnostic equipment or something like that.