I'll throw in my 2 cents, bearing in mind a couple things: 1) I don't have lots of money. 2) I am far less experienced than others here. My own system is not "high end," so a lot of my opinion is based on hearing stuff at friends' places, at audio shops, etc. and from experimenting with various inexpensive components (like <$300) in my own system.
Criteria here being that I think the current going price for overall enjoyability/musicality is inflated.
Overrated (aka: would still like to own it, but don't think it's at all worth the money)
McIntosh--it's been said, I know. But Mc is a cult, basically--with good reason in as far as they sound great. Price on some stuff is insane, though, $ per quality-wise.
Marantz & Sansui--Just by way of illustration, a Marantz 2220b routinely go for $200+. I would hazard a guess that any single one of us could put together a better system (especially using separates) for half that price from current local CL listings. As a matter of fact, on my local CL there's currently a kenwood basic c2 and any number of amps to pair it with that would come in under budget. Same goes for Sansui, though I think to a slightly lesser extent. As with Mc, really nice sounding gear, but not worth the price IMO. I basically feel this way about most receivers, to be frank, but even among receivers I think there are equivalent quality models from other brands at much lower price points.
Klipsch--Heresy, for instance, but among others. They sound nice to my ears, but only with some types of music, and they seem to add a lot of color to the sound, plus command a pretty crazy price much of the time. Most of the time, you could do better for the money, I think.
Underrated
McIntosh (again)--specifically, some of the AV processors. In general, the stuff without HDMI or 5.1 analog inputs undeservedly gets no love.
Advent 300--this thing gets plenty of attention online, but somehow has managed to remain dirt cheap.
Acoustat--not underrated in terms of respect they're given, but they can be had for a song because of their size and because people don't know about them as much as, say, magnepans.
Sanyo--because they've made looooots of garbage, a select few truly excellent products are massively underrated (some of the tp series turntables jump to mind, available on a reasonably frequent basis for under $50 around here with very good carts attached).
Hafler amps--to my ears, love them. I've only heard the dh101 preamp, which I thought was kind of harsh. The dh amps, though, are IMO a great value.
ARC SP9--Yes, I know, according to many they're not great unless mkii or mkiii and not nearly as good as the ARC tube preamps. But compare the sp9 with the MKIII pricing. Probably looking at like $500 price jump. SP9 and do the recap, you'd come out way on top.
JBL speakers--Granted, I'm not a big fan of the house sound. But still, for the price of a good many mid-level models around here, you could stack some Kloss-era KLHs (which I consider just "rated" in most cases, local pricing vs. the auction site).