I can't thank the Germans enough when it comes to naming things. The Unimog is a German acronym (Universal Motor Gerät Schnitzel) which translates approximately to "universal motor thing." Between this and the "bucket seat car," I can't keep from tittering.
Unlike the bucket seat car, however, the Unimog is a fantastically capable off-road machine with lots of military abuse behind it. Daimler-Benz originally developed the Unimog in 1950 (under French commission, if you can believe it) as a tractor, although the project scope was enlarged shortly thereafter to include general utility work, too. From its introduction in the early 1950s, the Unimog has been produced nonstop and somewhere over 300,000 copies have been made.
The individual feature and option list for the Unimog has always been exceedingly long and nearly all Unimogs have been built to a specific order. Militaries generally ordered Unimogs in large batches of identical vehicles (to simplify maintenance) but other than these, most Unimogs came off the factory as very different vehicles. A list of Unimog models shows that the variations of chassis, engine, cab, and other hardware are nearly endless.
Collectability: Average. A Unimog's value is directly related to its operating condition. Unless you've got one with amazing credentials (e.g., Charles de Gaulle personally spit on this Unimog!) then it's unlikely to appreciate all that much.
Here's why:
(+) It's really damned capable. Unimogs are designed and built to be extremely rugged. These trucks were designed back when Daimler-Benz had a very solid reputation for putting together really durable machinery, and it shows. Unimogs are difficult to break and generally easy to fix—you can buy a Unimog, use it to dirty duty, and as long as you keep the internals in good shape, never feel bad about it.
(+) It's a mix & match smorgasboard. If you need a particular piece of equipment, odds are that someone has bolted one to a Unimog at some point or another. It may take a while to find exactly what you're looking for, but if you really need a dump truck, people carrier, fire truck, or just a handful of power take-offs, then a Unimog would fit the bill.
(+) Two words: portal axles. This actually fits into the "capable and rugged" category above, but it's unique enough to mention again. The Unimog axles actually sit above the wheel centers, allowing greater ground clearance.
It's not powerful. I'm not counting this against the Unimog because although it's a bummer, it's not designed to be a fast truck. You should be aware, however, that these trucks weigh 6,000lbs and are often motivated by a mere 100hp. Don't expect them to be useful on the highway—it's likely that a Unimog will rarely see the top side of 50mph.
(-) It's a work truck. Since the majority of people buy these trucks to really use them, it's likely that prices will stabilize themselves at the low end of the scale. When supply runs out (or when industrial types decide en masse that the Unimog isn't capable enough for their needs) then prices will begin to rise—but since DaimlerChrysler continues to make these trucks in a format nearly identical to that outlined in 1950, it will be quite a while for that to happen. This, more than any other factor, keeps the Unimog from being a really fantastic investment.
(-) It's not worth it unless you're actually going to use it. Unless you're really going to use the capability that a Unimog brings, there are likely a dozen other vehicles that will do exactly the same thing more comfortably and for less money.
Expect to pay: $8,000 to $15,000 for an older model and up to $80,000 for a newer (i.e., not new) one. These vehicles were optioned and used so heavily that it's impossible to put together a good metric based on year and mileage.
What to look for:
- In general, vehicles with commercial service have been beaten heavier than privately owned trucks. Emergency vehicles are good buys, as well as low-use military models (i.e., radio trucks are good, water tankers probably aren't).
- Rust isn't typically a problem on the big mechanical bits, but do be wary of rust on the cab.
- Get a second opinion on any truck. Getting a Unimog fixed can be an ordeal: they're big and heavy and can only be fixed with big and heavy parts. This goes doubly when you're not quite sure what equipment is on the truck in the first place.

I'll second that (-) of speed and comfort.
Unimogs are REALLY slow and uncomfortable. They seem like great trucks to take snowboarding, but when you actually take one for a spin you see the negatives. An old Swiss army 404 can easily top 50mph, but it will take a long time and expect it to top out before it hits 75mph. There are the (+)'s for the Mog. I drove a mog around Silicon Valley for a few weeks and got tons of attention. It is hard to find a car that will do that in SV. Another (+): It makes the (real) Hummer look like a small little offroad toy. My last (+) is that most old 404 mogs in the states seem to have the radio truck box on back. They often come with chairs and a desk. You can fill them with computers and build yourself a serious wardriving spam sending maching.
For wardriving, I prefer a 1984 Plymouth minivan with Star Fleet Academy stickers on the back.
For the ultimate attention-getter, though, I'll always have to fall back on the Tatra 813...but good luck getting it around the drive-through at the In-N-Out Burger.