There are two overwhelming reasons to buy this book:
a) The updates that bring the GURPS Discworld or Discworld (HC) game-based information more up to date. Players of Discworld (powered by GURPS) or the earlier GURPS Discworld may or may not see this as worthwhile. No-one's Disc-based game is going to fall apart because they don't own this.
b) The scenarios. A GM who wants to get into the business of running Discworld rpgs but hasn't the experience or time to come up with compelling scenarios themselves would see this as an obvious benefit.
Discworld paraphernalia collectors might also want to own it for their own bizarre reasons. Collectors are loonies and there's no explaining their motivation.
Sadly, the Discworld RPG is long out-of-print and there doesn't seem to be any impetus to remedy that, so Amazon and online auction houses are your sources for a copy. I recommend Amazon because I trust a book dealer's descriptions more than those of some guy cleaning out his basement, and should - Offler forbid - anything go Wahoonie-shaped with the deal you have people genuinely interested in remedying matters and answering your e-mails.
Not only that, no-one is attempting to establish the out-of-print RPG supplement as some sort of new monetary standard by shill bidding the asking price into the stratosphere.
Back to "Discworld Also".
The tone is set by the title, which of course is a pun in the Pratchett mould.
The game system assumed is GURPS third edition. It is no big deal to use this stuff with the now-current fourth edition GURPS. Thank SJG design team for not doing a baby-bathwater thing there; Fourth edition represents a mere tuning-up of the previous edition. You could also run from this book using the GURPS Lite rules included in the back of the Discworld rule book, or do a port into an entirely different game system with a little work, since GURPS books are written in such a way as to facilitate that.
The material concerns various aspects of the game, from new and rethought templates for character types (such as young student witches, more hefty trolls and serious Wizards of never-let-the-players-be-one-under-any-circumstances-and-we're-not-kidding caliber) to the scenarios and campaign seeds. There's a lot of stuff on Fourecks (why the author still insists on using the more clumsy Ecksecksecksecks is beyond me, especially since much of the material is drawn from "The Last Continent" and in that book the place is referred to as Fourecks) which you'll either love or won't. There are a bunch of "Car Wars" puns hidden in that section too. You can't blame a dedicated SJG gamer for that.
[STRIKE]The artwork is good, but not of the same quality as in the original rulebook, nor is it used so effectively as a design element.[END STRIKE]
[EDIT:] That wasn't fair. The artwork is by a different artist and therefore of a slightly different style than that inside the covers of Discworld (powered by GURPS). Taken on its own merits the artwork is very nice indeed and is used quite effectively in the overall graphic design of the book. I must've gotten hit with the grumpy stick when I wrote that sentence. [END EDIT].
That said, Discworld Also is still a pretty thing to own and a perfect compliment to your copy of the Discworld rulebook.
Given the size of the thing it was obvious that this would be published as a paperback with a perfect binding rather as another hardback. Unfortunately, this means that after a little use (or even less AB-use) you'll be facing the prospect of "Autumn in bookland" syndrome, where the leaves fall like rain as you read from them.
My copy arrived from the vendor with a ding in the spine (probably transit damage) that pretty much guaranteed opening the book would destroy the binding (the pages scallop near the bound edge and this works like a lever to pull the pages free of the glue as you open them). Other 3rd Ed GURPS supplements I own have pretty much disintegrated after a use in-game. It's a fault of that particular binding and the tech used. All the contemporary game books that used it fall apart prematurely (GW rulebooks are a prime example).
I "solved" this problem in my usual heavy-handed way by replacing the spine and binding with a spiral binding. Now the book has no collector's value but can be opened flat at the gaming table.
Which was, after all, my primary reason for buying it in the first place.