The cap to luck is 650% by default. With Epiphany set, you can raise it to 812% though. Fortunate ascension perk makes it 200% more luck since it's above cap so 650% luck shows as 850% luck (200%+ is huge) and 812% shows as 1012% luck.
Is luck worth it? Yes. Its because it increases the rarity of all the gear dropped on the ground which in turn means more chances of legends, more rare gear to sell which means more gold sold: that goes great with Dealer perk which 1000% increases the gold sold or 10x, so if you sold a rare item that was 900 gold, it would sell as 9000 gold with Dealer perk. Best part is that Hirling dealer perk works well too.
Then more legends also means a higher chance of Eternal Drops and Crystal Legend drops even if you didn't equip Eternalized set, Crystalline set or Treasured perk from ascension. If you do equip those sets though, you will end up dropping many legends and eternals. Farmimg floor 200 quick with max luck, full inventory super fast with legends.
More legends also means you can salvage many for Dust which can be used to craft legends in Codex.
Eternals or crystal legends are great when converted at cost of 500k gold into an ultra rare crystal, usually ranging from Jasper to Obsidian. Topaz and amber sometimes included but rare as they are more common by other means anyway. Legend converting for 250k doesn't guarantee an ultra rare crystal like Eternals though so comverting legends can sometimes be a good way to gain lots of crystals or lose a lot of gold as well.
Item Quantity is Item drops and this is most important as well as Luck. More items dropped means more legends, higher chances of an eternal being dropped, more general loots, more gold and most importantly, more crystals and mythstones. High item drops is very important for farming floor 500 for ultra rare crystal drops, farming for a huge bunch of gold , mythstones if short on them and finding legends, especially the hard to find legends.
Combine Item Drops with Luck = a lot of legends.
There's also somethng about how Luck can affect rerolls when you add an affix along with 25% item drops, which probably explains why i get perfect items if i make a build using my farm characters and then transfer it to other character classes or my main character. It seems like just RNG but Luck is in the name, to be lucky.