On plain vanilla older stuff it's a useful reference, on key issues & books after 1990 it's of little use. Mile High is the market leader & is much more on top of things, they also have list prices for all grades even if it's not in stock. Their demand is such that they can charge pretty amazing prices but if you take about 1/2 their list price & adjust a bit depending on the # of copies in stock you'll get a pretty good average retail figure. Hulk 181 (first Wolvy) is a silly miss, guide in GD is $64 but it'll sell on Ebay for $300, only the most important book of the bronze age. It's time to get with it on Deadpool & Spawn too, some list for $3 but sell for more like $30-50.
Seems to be intent on marking down mid-grade values but don't let it fool you, comics are hot & most are going up in all grades. I think it's trying to reflect the fact that most sellers have to mark down books that aren't CGC'd, sellers please note that 90% of books bought online are either Overgraded or arrive damaged so of course I'm not paying full guide. Sellers with a good rep have trouble keeping quality material in stock, somebody please sell me some.
In all fairness Bob Overstreet is like 75 or 80 and this is a huge market, no way he can keep up. He should skip all the new stuff with all the variant covers & stick to pre-2000, either that or let someone else take over. Other comment was correct, there's lots of post-1980 stuff that just needs to go on a "cover price or less" list.