are they bad regulators? no
are they a bad decision to purchase? yeah....
They're very expensive, do have a history of discontinuing parts availability *the Dacor mentioned above is probably the largest uproar on that*, and they really just don't offer that good of a value. If you're in Italy, and that is what all of your buddies are diving, then they may make sense due to those extenuating circumstances, but if you're in the US especially, they are one of the last regulators I would recommend, and here's why.
Their "premium" reg, the Abyss 22, starts at $600 and goes up to almost $700 for some versions, and their cheap one is $200 give or take. That is just for a first and second stage, no pressure gauge, no inflator hose, no secondary. Their cheap reg, Prestige/Instinct/Rover with the 12s first stage all come with a really cheap first stage.... Will it work? yeah, is it quality built? probably, does that make it worth buying? no. These are all prices from Leisurepro so are going to be as cheap as you can get.
Now, what else can I buy for $200? Not a whole lot, but you can get the D1/Classic from Hog or the DC1/212 from Hollis which upgrade you to an environmentally sealed swivel turret with 5 ports that makes hose routing better. So that's out, but if you're buying super cheap regs of that design, just grab an old Sherwood, it's better anyway.
Let's look at the high end Abyss 22 which goes from $600 up to $675, what else can I buy with that? I can buy a full streamlined OW set from Dive Rite with matching second stages, an inflator hose and an SPG on the swivel turret and both the second stages are reversible at $600, at just under $700 I can get an extra first stage to carry in case of a failure so you don't have to get it rebuilt while travelling or as an option for if/when you step up to a pony bottle, doubles, or sidemount. If I'm feeling really spendy, I can go up to $800 and while you're spending $200 more than you would for just the Mares Abyss 22, you can get a Poseidon Xstream singles set with matching second stages, the MK3 first stage, and SPG. One of the best regulators ever made period. That price range also opens up Scubapro, Apeks, Atomic etc etc, all much better regs than the Mares...