What do you like best about Mezmo?
Very easy to implement. 
Define the name of the app and environment.
App is great! Compact, good UI. Logs are easy to read. Different color for warn/error. Visualization of spikes.
Setup alerts, including alerts when certain logs spike in frequency. Many great options in the app.
App makes it easy to see only certain types of logs, from certain servers/apps/times. Even search the logs!
Generous amount of data retention per dollar, and unlimited number of hosts. Minimum charge is $10/mo.
Mezmo does not offer any services other than logging. Other cloud logging solutions offer all kinds of performance monitoring. But for me this is actually a benefit. It makes the app very simple and easy to use and SDK very simple to set up. The more complex tools are often too cumbersome and awkward because they do too much. Do one thing and do it well. Review collected by and hosted on G2.com.
What do you dislike about Mezmo?
* It's not possible to submit regular "log" level. You have to choose instead from "debug", "trace", "info", "warn", or "error". When attempting to log {level: "log"} in NodeJS, I get an error. The SDK simply does not allow it. So, it's annoying when logging regular "console.log" level, I have to submit it as {level: "trace"} instead. It's not a big deal for me, because I don't use trace.
* Would be nice if debug/trace/info were different colors in the app. Currently they're all grey.
* I don't know if they fixed this or not, but as of January 2021, there was a limitation... If you make a log less often than once per minute, then their logging service seems to go to sleep. Then if you haven't submitted a log in a while, it just would drop the first log, not report it at all. This was frustrating. I fixed it with a "setInterval" on my server - submitting a "debug" log once every minute. This way, even if I send occasional rare log events from different servers, all the logs now show up without getting lost. It's easy enough. However, in order to not pollute the app dashboard, I had to set aside "debug" level for this purpose, and never use it in my actual app. Then filter it out from the app.
So now, I'm able to use (and see in the dashboard) "info", "warn", "error", and instead of "log" I have to use "trace". These are the only levels available that actually work. 
Fortunately, for me that's enough. And, the UI is very good, and the price is very good for how well everything works. But if the service was less user friendly, or price higher, my rating would be lower. So overall great job!
* Documentation is not well worded, not easy to browse. Documentation mentions a "fatal" log level, but that doesn't actually work. Review collected by and hosted on G2.com.