If you visit this time of year, you'll never want to leave.
I lived there for many years. It's a fantastic city. Most of the good riding is in the north/eastern parts of the county, where it's a mix of burbs, ranchettes, and alpine/desert vistas. Pretty, but also really really busy. It's very hard to ride anywhere without biting off big chunks-o-slab. There is very little driving on surface streets there- you just can't get anywhere like that.
North County between Del Mar and Carlsbad are still some of my favorite places, and PCH/101 rolls right through them, right next to the big blue ocean and some of the finest beaches in the US.
Coastal temps are mostly cool. Inland it gets blinkitty-blink hot in summer.
Mexico? Very questionable at the moment. Drug-related nonsense on the Mexican side has made travel south from TJ highly suspect. (There was just a huge shootout in TJ this weekend?) Further down it might be safer, but the Frontera is a bit like the wild west @ the moment.
Also, I mention this because It might be important to you- The restaurant scene in San Diego is downright pitiful for a city of that size! Be prepared to do a lot of work sussing out good places to go, only to have them disappear overnight. But there hidden gems too. There are worse problems to have.....
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