Helium is too thin - it will escape the tire more quickly than air.
I don't know that the comment about Nitrogen not expanding or contracting is correct - everything expands or contracts compared to relative temperature. The idea behind using Nitrogen is that the gas is thicker than plain ol' air and therefore does not escape the tire as easily as air does.
Of course, the other comment above is correct - air is mostly nitrogen anyways, so what difference does the extra 20% Nitrogen give? I'm not sure, but it costs an extra $20 bucks!
Most places are offering the Nitrogen and then claiming that only they can put more Nitrogen in your tires, thus forcing you to return to their shop and their shop only, thus giving them more opportunities to sell you stuff.
I say "ignore the hype - this is no-big-deal."
Jeff