I had a few. They all had problems.

I had to remove the battery and insert it into a charger. That's not awful, but being able to plug a cord right into the unit is much nicer.

I had to try lots of mounts. Most rattled or went out of adjustment or just aren't well made.

Beam shapes are generally horrible. Usually, there is a hot spot in the middle of the beam. My eyes gravitate towards it, even when it is not illuminating the important stuff.

Battery life is unpredictable.

Information on how bright they are is bad.

Some of the lights failed, especially the expensive one.

A light designed to be used on a bike is better for me. The mount is designed for the rigors of riding a bike. The beam is more evenly dispersed. Battery life is similar to claims.

I have a B&M USB rechargeable headlight (IXON Core). Beam shape is excellent, and battery life is amazing. It doesn't flash. The rubber band mount is a little too loose. Construction otherwise is excellent.

I have a Cygolite USB rechargeable headlight. Lots of good modes. Beam shape is OK. Battery life is good. Construction is excellent, and so is customer service.

I have several dynamo powered setups, and I like these the best, though output is lower. Beam shape is excellent. Construction is excellent. Battery life is not a question. The lights bolt onto the bike, so convenience is maximal unless there is a risk of theft, but I haven't had a problem locking the bikes up outside. Reliability is maximal. Cost is high. Weight is low. The drag from the hub is imperceptible. For easier setup and lower cost and lower weight, you can use a sidewall dynamo, but drag is perceptible, and it's noisy.

If I had bought the expensive stuff in the beginning, I would have saved money.