THE DIMENSION RIDERS is one of those stories that's quite a bit of fun while you're actually reading it, but which doesn't particularly stick in one's memory. There's nothing inherently wrong with it, and, in fact, there is quite a lot here that is very enjoyable. However, there's also nothing really fantastic about it either. I read the story, and I enjoyed it, but there was very little that stood out about it in my mind after I had finished. It works well as a simple adventure, but don't go into it expecting anything more.
The plot appears to be a tad more ambitious than it actually is. It's set in two main time zones with a lot of interacting going on between them, and that layer of complexity really doesn't have all that much impact on the plot. There didn't seem to be any real plot reason for having the action spread out over time rather than space, but it did make the story seem just a little bit more interesting. It's a case of style over substance, perhaps, but it works. The plot may not be especially complicated, but it is fairly clever in a few places. There are parts that are a predictable, yet I found one or two twists that genuinely surprised me. The modern-day Oxford setting is drawn realistically and goes a long way in helping add to the atmosphere. This contrasts well with the ghost story feeling of the passages set in the future.
The characters introduced here are competently drawn, but aren't particularly deep. They fulfill the functions that the plot requires of them, and aren't really fleshed out beyond that. Again, the characterization isn't anything that really hurts the book, and, indeed, there are a handful of moments that do stand out as being special. It's just that the vast majority of what we see is adequate, but not great.
Actually, my overall opinion of THE DIMENSION RIDERS was adequate, but not great. There is indeed a small smattering of interesting pieces, and the storyline itself it quite fun, even if it isn't terribly deep. The hints dropped here concerning the ongoing Alternative History story-arc are intriguing. You could probably skip this one and not worry about missing anything terribly exciting, but you'd be missing out on an enjoyable few hours.