Simple and "easy" to use.
The 3rd party website containing the userstyles is awfully slow. (This didn't influence my rating)

Slowpoke... Takes about 5 minutes for it to understand that the ONLY style actually DOES exist. Until that it strongly believes that there is absolutely NO styles.

been using this a long time and it is such an excellent extension that, given its syntax highlighting, checking and auto-complete, can help one learn css in addition to applying it

and here i'm adding a 6th star just because of it perfect privacy policy...

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Great addon, but has it's limits. Also userstyles.org page works very poor, so getting styles is very hard.

It does the job.