Nirnroot DOES respawn, folks, despite what the loading screens or what-not say, so you can go to Sarethi Farm (or any other nirnroot spot) every several days and collect it forever. Nightshade and Deathbell aren't that tough. It's just a matter of poking around where they're common; Deathbell in the swamps around Morthal are common and easy to spot. Just watch out for the spiders and occasional chaurus.

Over the long game, yes, it's worth it. I'm lvl 42; I just reloaded and checked the chest a few times, and it tends to contain at least a couple of endgame quality potions (Blacksmith's elixir and the like) plus some other stuff that's worth decent coin if nothing else.

If you're not running the fortify enchant/alchemy/resto loop, those are good potions. And they're FREE. All you have to do is let the chest refresh, bring the key, and grab 'em.