Better brakes?
 

Is there any improvement to be had with aftermarket brake pads for v-brakes?

Maybe something like this? http://www.rei.com/product/767427

Or is it just a waste of money?

Kool Stop Dual Compound Mountain bike pads. $10 per pair and worth every penny. Look like this:

http://tfe.me/image/480_345/BR1227.jpg

Is it a noticeable difference from Shimano OEM stuff?

Also, is it worth the extra couple bucks to get the 'reloadable' type with replaceable pad cartridges?

eh, just went ahead and ordered some of those kool-stop pads, I figure at that price, it can't hurt! :D

The Shimano pads are very decent stoppers. Kool Stop is better.

But, the Shimanos are worth wearing out first, as they are decent.

This has been my experience, too. Keep the original pads around as spares, since you've already ordered the new ones, but I think you'll notice a big difference. set them up carefully, of course.
BTW, do you mean you ordered them online? You'll pay seven bucks shipping for something that would have cost maybe two dollars more in a bike shop, and you'd already have them on the bike?

If his local shops are anything like the ones around here, they wouldn't have any in stock. It is easier to pay a couple bucks shipping and have it at my door in a couple days than to drive an hour, be told they don't have it but can order it, then drive an hour (again)to pick them up in a week or two.

Those Koolstops & keep the rims wiped down with alcohol, etc. Great difference !