I can't answer about the coverage in San Diego... Coverage is great in Utah, but we've had cricket for 5-6 years now.. San Diego being a newer market, it may or may not be as good... If no one can give you a good answer here, I'd go to your cricket store and find people paying their bills, and ask them about it.
The 50 plan will work if you're travelling to an area with cricket service. It does not include any roaming. You can purchase roaming minutes in bundles. $5/mo gives you 30 min, $10 gets you 70 min, and 15 gets you 120 min. Roaming minutes are only used when you're outside of the normal cricket calling area in any market. Alternatively, you can upgrade tothe $60 plan, which includes 300 roaming minutes. However, you don't get web access with that plan.
To clarify, if you come to Salt Lake City, your phone will work, without roaming (as long as you're in the salt lake city cricket area)... If you go to North Dakota, you'll be roaming, because cricket doesn't offer service there.
Sorry if I'm explaining stuff you already knew.. I just wanted to make sure it was clear.
