The bad things you have heard are probably the very early ones when Sega teamed up with D3 to do cheap remakes of classic stuff. The results were poor, and starting with Virtua Fighter 2 (Vol16), Sega decided to put more effort and produce them internally. M2 wasn't brought in until Space Harrier II (Vol20). Every one of the M2 developed 2500 releases are amazing, and the ones which are collections are really value for money.
M2 developed Space Harrier II (20), SDI & Quartet (21), Last Bronx (24), Gunstar Heroes Treasure Box (25), Tetris Collection (28), Monster World Complete Collection (29), Galaxy Force II Special Edition (30), Virtual On (31), Phantasy Star Complete Collection (32), and Fantasy Zone Complete Collection (33).
While M2 did not work on them, Virtual Fighter 2 (16), Dragon Force (18), Dynamite Cop (26), and Panzer Dragoon (27) are pretty solid releases as well. I haven't played every single one, so there might be even more gems there depending on the titles you like. Just stay away from anything before Vol16.
They all come with really thick full color manuals, and are packed with bonus features, all domestic and foreign versions of games when available, support low resolution modes, etc. The 2500 series on the PS2 from Vol16 onwards is basically the Criterion Collection of Sega classics in terms of physical releases.