I live in San Diego and we just went out to Anza-Borrego yesterday. It was nice, but I don't think I'd travel cross-country just to see it, although as others have pointed out, there's lots of other great things in the area that would make a trip worthwhile. If you do go, be warned, it is RIDICULOUSLY crowded out there. We left around 6am yesterday, got into Borrego a little before 8am, and it was quite busy already. We left town around 10:30am and the roads leading into town were backed up for MILES. People were parking miles away from the visitor's center and walking in (even though there were better areas to go see the flowers, they just didn't know where they were going). So, if you do go, I suggest:
1. Get there early, leave early
2. Make sure your gas tank is full each morning
3. Pack a lunch, you'll waste hours if you try and go to a restaurant in town
4. Its the desert - common sense rules apply. Bring lots of extra water, sun protection, watch out for rattlesnakes, etc. Its already in the upper 80's to 90s during the day, and there is NO shade.
5. The park is HUGE, do some research so you know where you want to go ahead of time. The bloom is fantastic in Coyote Canyon and along Henderson Canyon Road, but the crowds in those spots are enormous. If you can get a high clearance or 4WD vehicle you'll have more options of where you can go.