Black Lion Chests are lucky packets, basically, that can only contain Boosters, Services, Consumables, Backpacks, Miniatures, Tonics, and Everlasting items.
From a single chest, you'll always get three things: one or more boosters, and then something from the other categories, like Services (Bank Access Express, Instant Repair Canisters, etc), Consumables (Black Lion Keys, Salvage Kits, Communal Bonfires, Luck, etc), or things like the Quaggan backpacks, Minis, or Tonics.
The *really rare really valuable* stuff are the Permanent Bank, Black Lion, or Trading Post Contracts. If you ever get one of those, you hit gold.
When I buy the Black Lion Keys, I'm doing it primarily to support ArenaNet. They've never asked me for any money after buying their game, and I respect that. The stuff you get out of the chests can be handy. Revive Orbs are very useful, Repair Cannisters can save your bacon in Fractals, and so on - so I see them mostly as 'nice things', but none of it is critical in any way.
Getting a Self-Style Hair Kit or Total Makeover Kit is nice though, since it gives you access to the extra faces and hair colours you can't get during usual character creation.