"Imagi Nation" contains a clutch of good, interesting ideas for a novel RPG set in the modern-day British Isles, whose main island has been beset by a cloud of dream-nightmare effects. The "Description System" game mechanics are innovative, as detailed by earlier reviewers here, although as the adjectives and adverbs used in it must be converted to number bonuses to a D6 roll for game-use, the difference to the commoner numerical-skills-based RPGs is less than it might seem initially. The book has plenty of mysterious atmosphere about it, including flavour paragraphs presented as "postcards from the front" and illustrations, all of which is useful for players and GMs alike.
However, GMs have a lot of work ahead to turn the ideas here into a proper RPG campaign. Some examples. Remarkably little information is given on the origins and nature of the dreams-become-real effect, and how it may be countered by the PCs. There are no maps of where things were, and still might be, across the British Isles, a particularly surprising omission in an otherwise well-illustrated book. PCs must possess creative talents and suffer from an active mental illness to combat the effect on the mainland, yet these key talents are allowed from a quite restrictive list of arts-related options only. For instance, architects and sculptors are there, but not equally creative mathematicians or builders, a distinction in need of some justification in game terms.
Overall, the concepts presented are fascinating, and could be readily adapted to different times, settings and game-systems, perhaps most workably for a greatly-reduced physical area affected. Had this book been presented as such a background setting for other RPGs, I'd have rated it more positively. As it is, it's too much of a players' guide, with too little help for GMs, to be classed as a full RPG. With regret, because I do like the ideas here, I've marked it down accordingly.
[3 of 5 Stars!]