I'm sure there are people around to whom a $100 or so doesn't matter, and to them -- if they want to use Mandriva -- it's probably not a bad idea to buy the thing. But if you're short on money or just like to know what you get with your money (i.e. don't throw it over rail), I'm not sure if you get that much out of it. If I was to pay for a Linux, I'd pay for a server package that has a consumer service for the server needs in the price. For a desktop, I would not pay (not that much, that is): the manual contains information that is probably just as easily found on the web, most of the programs on board are on the web etc..
The benefits probably are, if you're having trouble you could ask somebody who's paid for answering and get an answer without needing to wait if somebody here at LQ cared to answer
I guess that's the main point (if it's bundled in the price). The manual probably can't be That Good, unless it's bible-like overall answering miracle; however, for a new person to Linux it could be a very good idea to have a physical box of paper and ink to read when things go wrong. Windows costs you money (more than $90 even) and what do you get with the price? An operating system that you could have for free (multiple alternatives), a small light manual (more information is freely available on the web) and a glittering licence to show the FBI when they come to you suspecting you've pirated MS property
and that's pretty much it. You can't even go under the hood and change the system to your needs, because the licence says you can't. With Mandriva things are a bit better (lower price, you can actually change the system at least to some extent without anybody going mad, you get more applications than just the OS and basic tools, ...)
Conclusion: if you are unsure about your skills, may "need backup", have $90 money you don't explicitly need and are in a good mood..go ahead and buy it. I don't think it's a bad deal then. But if you think you know the manual stuff without the manual, you don't want to pay another $90 in a year or two when it becomes old and are overall unsure if it's a right thing to do, then don't buy it. And the most important: because there exists a free version of Mandriva too, try it before buying the product to see if you even like it