Potion!

Is it worth buying health and mana regeneration potion? I mean they are quire expensive some of them at the market and I don't really like the fact that they got a max cap but they still do thier regeneration of *healing* wheather it's mana or health.

For solo play they can help. You shouldn't need them in dungeons unless you decide to run without a healer.
For ethers, I have caster friends who like stocking them just for replenishing quickly after switching classes/dying.

For hp pots, it is for that "oh shit" moment when your healer is not paying attention.
For ethers, BLMs can use the +270 mp one to add an additional Flare in their rotation once every 2 min. WHMs will find it useful too for that additional 1-2 cures when times are tough.

I didn't use a single one of these until I started doing Coil and even then, I only use Ethers to help keep Mage's Ballad up longer as a Bard.

They have their uses in my opinion.

Health : solo play mostly
Mana : always useful, additional flare as mentioned or long fights
Stat boost : the most underestimated potion. DPS race? pop a STR, DEX, INT (MND if cleric stance) mega potion. You know you will get a big nasty hit as a tank, pop a VIT potion, healers will top you then.
Status cures : these are situational.(works for an hydra fail and you get paralysis)
Attack potions : they have their uses, could even be a fail safe for example silencing potion for ADS

They are mostly situational, but I believe SE should separate the cooldown timers by category