Are these manufactured houses nowadays as good as a traditional contructed house? I'm considering having one of these pre fab houses built on my lot .Are the savings considerable or am I asking for trouble in the long run ? I'm in the S.F bay area where the neighborhood houses are in the $700,000-million dollar range .The city I live in does allow for a pre fab to be built Thank you

A prefab house is built in a factory on jigs likely meaning a straighter, truer, tighter house. The building is much more organized, and important details are much less likely to be forgotten. They are very sturdy too. I know some people that have one, and they are happy with them. I would go for it.

The manufactured homes of today are much better than the trailer homes of yesteryear.  However, there are some things you need to consider.  Some companies can and will cut corners.  Some won't use solid 2 x 4 studs but 2 x 4 finger jointed studs and 2 x 3's on the inside walls.  The Fixtures used may be second grade.  The wall board is usually only 1/4 " thick.  You can order your home with upgrades such as 3/8 or 1/2 inch sheet rock.  Get double pane windows.  Be sure the roof is 2 x 6 and that you have the proper Seer Rating for your AC Unit.  Insist on Plywood Flooring Not Particle board. ( a small leak will turn out to be a Nightmare).  You will need to check thoroughly for flaws as the people who assemble these homes are usually unskilled labor working under a skilled forman.  Walk around with a tape measure and a 4" Level when checking.
go out and Look at some homes and talk to owners first! I know the price can be attractive but remember that the Resale Value doesn't go up like site built homes!  Whatever you decide don't buy a used one!