In my pre-flashaholic days: had a M*g Solitaire on keys, but it got completely flaky, so I replaced it with a Photon II (green led). The Photon was my keychain light for many years. Its original battery (1xCR2032, they switched later to 2xCR2016) is still going strong.
When I found CPF, I got an Arc AAA and added that, later putting it on a BP Mini-clip. Also, added a Photon II with red led for night use, giving three lights on the keys. This was with a multi-ring key bundle that was already pretty large, so the lights didn't make it that much heavier.
Decided I had too many rarely-used keys, so removed a lot of them, consolidated all keys to one split ring. Added a Spyderco Jester knife and matching Photon II (green case, white LED, Spyderco logo) and removed all the other lights (Arc and two Photons). That was a huge improvement, made the keyring much smaller and lighter. If I expected to need a light for something, I stuck the Arc in my pocket, otherwise left it home.
When Arc went out of business, I put the Arc back on my keys as sort of a tribute to them. It's still there, but I'm getting ready to take it back off again. A Photon II will run for quite a long time if you don't mind it getting somewhat dim.
Also, I recently got a Photon Freedom ($12.50 sale at Brightguy). In low-powered mode the Freedom should have enormous runtime, much longer than the Arc AAA, though of course it's much dimmer in that mode. But I don't think I'll carry it on my keys, because I like the green-cased Spyderco-Photon II that matches the pocketknife.
Kevin L is selling some very close Photon II clones for $1.25 each. These are a great value, nicer lights in my opinion than the slide-switch Countycomm lights that were my #1 bargain til these P2 clones came along. Except for lack of waterproofness, the P2 is my favorite keychain light: very small (smaller than other coin lights), simple, no computer electronics to fail. If it gets soaked just rinse it with fresh water and dry it. It might need new batteries but otherwise nothing can really go wrong with it.