The way I see it, everyone has to take a beating sometimes.
High Density is a soak mutation that adds Beat and Bullet soaks. It's also the prequisite for Lithodermis.
HD is in Maas Neotek, at the Medgentech Kiosk. See MK on the map. From there, and head down. You may run into hepcats in this area, which are dangerous to low-level players; usually, it's possible to run past them.
This mutation grants:
+3-5 beat resistance +1-4 bullet resistance
- 3-5 beat soak. Beat is a mildly uncommon damage type in PvE (outside of the early game), but does crop up with basilisks and some other mobs. In PvP, beat is much more common, so this soak is medium- to high-value.
- 1-4 bullet soak. PvE bullet damage is rare outside of optional areas such as the Skydock and Glowstiller, but in PvP, this is probably the most common damage type in the game. However, when you are facing guns you'll either be armored to the teeth (making 1-4 superfluous), or else caught off-guard without soaks or with elemental damage (in which case 1-4 won't do much), so this is a low- to moderate-value soak.
High Density has no disadvantages aside from taking up a mutation slot and one of three available soak mutation slots.
High Density does not make any changes to a character's appearance.