The price point definitely can't be beat, but you're not getting a new product for $50 (I assume that Ebay sort of pricing). The power/heat debate depends on the boards you're looking at. The VIA ones do offer a lot more horsepower (you're talking in the Ghz range), but take more power and require heat sinks/etc. On the opposite side of things, the Geode based systems such as the ALIX boards are on the same power/heat levels as the slugs and routers (MIPS based); my Alix 3c3 runs at 3W on linux idle and about 5-6W with the Wifi radio running in AP mode (the wifi card is a high power card though which is NOT the same as you get in most wireless routers) and a few servers and things running on it. No heat sink required, and the Geode will even give you thinks like hardware encryption support for added throughput, etc.
Plus you're talking 256 mb RAM, a proper CF card for storage (or IDE adapter if you want), and real USB 2.0 throughput (not sure about the slug, but I know on the Asus routers although it's a USB 2.0 port, the CPU simply doesn't have enough power to give you maximum throughput... try copying a large file to the slug and see how long it takes compared to copying that same file to the drive connected via a USB 2.0 port on your computer... you'll see the difference, and it won't be because of the network speed). You can also get with sound options, etc. Again in my mind it's a nice alternative/step up from the slug if you need a bit more performance while keeping the original benefits. (The various options for ethernet ports VS mini PCI slots VS other ports such as audio/VGA/etc is also pretty handy).
In terms of cost, it's a bit more expensive than a $50 slug, but you have to remember too that you're buying new, still manufactured product and you're getting something with a bit more kick to it. In my mind the pricing is on par with Beagle or any of the other modern embedded system boards... again just with a different purpose/target in mind.