-- Last Updated: Oct-22-12 3:29 PM EST --
I second what guideboat said. Aluminum is, obviously, not a first choice hull material for the avid canoeist, however if it is your first boat, you have a limited budget, or want a durable/minimal maintenance hull, Aluminum boats often work fine. Their main drawbacks are the typical 80+lbs weight and (my non scientific opinion) that the hulls have less engineering in them. By that I mean I dont think the typical Grumman or Alumacraft has consulted much with respected designers like Kruger, Jensen, or Bell for their hull design. That said, I have paddled alumacraft canoes up until recently and think they work well for what they're designed to do for and they're cheap.
I have watched canoes all season on craigslist and seen a few grumman's go for $300 in good condition, so thats hard to beat if you're just looking for a cheap boat to get you on the water. And like GBG said, you can probably resell that $300-500 boat for the same you paid in a year or 2.
On the other hand, if you're patient, look often, and are lucky enough to catch a great deal in time: I saw a Rx Spirit II for $600, a Tuff weave Spirit II for $600, a MNII Kevlar UL for $650 all in good/great condition on craigslist this year. All those are less than 30% of original MSRP, so if you can wait you can find smokin good deals on intermediate/top end boats too.
So all in all, you can buy an aluminum boat, use it, see if paddling is your thing, and sell it if you lose interest or want a nicer boat, with little or no downside, so why not? If you think you'll be into canoeing you can watch CL and find a good deal on a nicer boat too.