Started with a cheap pulse oximeter. It was helpful information, but not as practical to use for pacing as dedicated heart rate monitors (if you can afford the cost). I have really liked the help documenting my sleep and total steps in a day. For pacing after a prolonged increase in severity, I have found a dedicated heart rate monitor to be very helpful.
I also use a Polar A360. A key factor for me: I can turn off the radio on my Polar A360, as I'm electrohypersensitive. It also allows syncing/charging by cable connection to computer (so don't need wireless at all). It is sufficiently water resistant to help pacing in the shower (when to sit). It seems reasonably accurate for catching when and how much I sleep, as well as picking up total 'steps' (counts steps even when not recording an event). I only truly track my major exertions (shower, take dog out, etc). Also, as I do similar activities from one day to the next, seeing the increase in heart rate to do a typical activity tells me to back off as I'm close to (or past) the point of PEM. A device that can potentially warn of PEM has been very helpful.
I find the monitor good to slow me down when walking, and to sit more when showering. I wear it tightly during an activity, but more loosely otherwise, and find it relatively comfortable.