I moved to Foothill as a renter in December, having lived previously in Fountain Valley, Costa Mesa, Newport Beach forever.
I have to say, I love it so far. I really enjoy the topography of the place. All the hills and grade differences guarantee green areas, spacing between developments, and lots of views. The hills behind us are beautiful, and they keep me from feeling "boxed in" like I did living in the sprawl below. I also took up Mountain Biking when we moved to the area and fell in love with the sport. The trails are filled with friendly locals, hikers, bikers and a few on horseback, and its fairly easy to get far enough back into the canyon to actually feel "away from it all," even with a simple hour ride from the doorstep.
The drive down Bake moves quickly, but is getting somewhat old. I'm too cheap to take the toll road unless I'm in a real time-crunch (it probably saves 10 minutes if you're coming from north 5, more if you're heading to RSM). The elementary school is very good, as is the local pre-school, but having to drive to RSM for middle school is a downer. Trabuco high is closer, but still not within bike riding distance.
There's plenty of local shopping, as noted above. We've never been to Walmart, we choose to visit Target for the same purchases. There's a fine array of chain restraunts, a good cinema, and a new cool sports bar, Rudy's. One thing I like about being so far off the beaten path is that the people you run across are generally neighbors. Growing up in Newport/Mesa and around South Coast Plaza, I've grown tired of living where other people vacation.
We're renting in the Vineyards, have a nice 3 bedroom townhouse with a yard and facing a vegetated slope. It's private and quiet, most of the neighbors are young and friendly (white, indian, asian). I can't comment on the "its soooo much hotter and colder in Foothill" since we've only been there since December. It certainly wasn't any colder than the marine layer winters I had nearer the coast. It hasn't been hot yet, and to my surprise there's been a great breeze blowing almost every evening.
We rented here with the intention of "try before you buy" while always waiting out the market insanity. If prices continue to fall enough, we'll try for one of the larger homes above Portola. Otherwise we're eyeing the same general location, but further down El Toro, Lake Forest Blvd, or Bake, staying inland of Trabuco Blvd.
Those are my thoughts anyhow. Looking at the above posts I realize this city is not for everyone, and I'm really ok with that.