Yeah, USB rechargeables taper off in runtime per charge, gradually at first, then suddenly at end of service.

Runtime per charge, when fresh and over time, vary significantly. I suspect most light engineers are constantly tweaking their designs, taking advantage of the latest tech that's still cost effective. They have to meet a price point that's increasingly competitive. If price weren't a factor they could probably boost overall performance and runtime considerably, if customers were willing to pay closer to $200 per light.

Check the "We test lights" website for comparisons between manufacturer's specs and real world performance. The site is run by Light & Motion but they aren't biased. Some of their competitors actually test better than L&M lights, but they don't fudge the data to skew results in their favor. And there are significant differences between different models within any brand. Very interesting stuff.

My Ion Speed Pro camera got more than the rated 90 minutes at first in July two years ago when I got it. I run it on almost every ride. Now it's down to about 80 minutes per charge, although that depends on the temperature. In the recent 100+F temps it ran closer to 80. On cooler days it runs closer to 90. Not bad.

My Light & Motion Urban 500 performs almost exactly according to manufacturer specs, even after 18 months of regular service. Excellent lights.

I recently got a NiteRider Lumina Micro 750 because it was discounted so heavily at REI. I like to buy locally because they let me test the lights. Same beam pattern and overall characteristics as my L&M Urban 500. But in actual practice the output tapers off very quickly and within an hour this particular NiteRider model on high is only as bright as the L&M Urban 500 on low. And that matches the real world test reviews, so it is performing as tested. Just not up to the same standard as L&M. However other NiteRider models do perform better. And I still consider the Lumina Micro 750 a good value at the discounted price, so no real complaint. I just run it on medium as a compromise between brightness and runtime. And I like the NiteRider's rubbery weather-resistant USB cap much better than the Light & Motion's, a plastic thing that broke off.

Serfas E-Lume lights are a really good value now that they're heavily discounted. Good performance. Weird mounts. Be sure not to overtighten the adjustable clamps or the pins will tear through the stiff rubber straps. The camming clamps can exert a lot of tension.