Yes, the OG progresses from easy to difficult as you proceed through each section. That was not true before the 11th edition- if you have the 10th edition, the difficulty is random- but in the orange 11th edition, the quetions get harder as you proceed through each section. I'd add that the hardest questions on current real GMAT tests are more difficult than what you'll find in the OG. Unless you're aiming for a score above the 85th percentile or so, this doesn't much matter, but if you are hoping for an elite Quant score, GMATPrep and GMATFocus will give a very good indication of what to expect at the most difficult levels of the current GMAT.

I won't comment on specific companies, but I agree that many GMAT test prep products are not worth spending time (or money) on. As with any other business transaction, I would recommend to anyone that they confirm, by reading reviews and testimonials as one finds throughout this forum, that a product is worth investing in. The GMAT market is saturated with products, many of which are not especially helpful (and some are even counterproductive). By reading this forum, you will find many recommendations for the good prep materials that are available.

The OG supplements are very much like the OG -- they have different questions from the OG, but the difficulty range is similar. Both are excellent sources of practice questions; just be aware that you won't see many questions that are truly at the 800-level; you may need to look at GMATPrep or GMATFocus for those. But unless you're already at the 48-50 level (scaled score) in the Quant, the Official Guide and the supplement should give you plenty of useful material to work on.