Well I can say they do work and do actually give the HP that is advertised. I personally don't think the weight difference is worth it. I would only pay the higher price for aluminum if you care more about 'show' than 'go'. I have the cheapest set out there I think (Mac Performance) and they look just as good now as when I bought them. Some say they rusted on them. I don't live in a area where rust is common so no problem here.
I always recommend the aluminum over the steel for two simple reasons... they will not rust and they are a little lighter than the steel. Is the weight enough to make a difference? That's up to you, but when many of us kill ourselves looking to drop as much weight as we can, we realize that its a lot of "little" stuff that adds up to big weight loss.
I always recommend ASP aluminum pulleys, they can be had for right at or under $100. I've had my set on my car since 1998 and I haven't had ANY problems with them and they still look like the day when I bought them. They also took my times from a 14.96 at 91.5 to a 14.6 at 94.89mph, that's a GREAT band for the buck.