We have used CM's that are very small shops to the
very large.  IN all cases, the first boards thru,
complex or not, require you to hold thier hands in
order to make sure it gets done right.  Once they have
a process down, its usually pretty much...build me 100
of these units.

Small CM's will sub the board out, even get them from
overseas if qty is enough or build themselves (if they
are a large CM that is...one of ours is)

Testing.  If you want them to test, then its going to
be a cost adder.  They typically will want the test
setup (we provide the test jig) and depending on the
design, rework can sometimes be easier at the CM
facility (large BGA's are usually not for the faint of
heart).  If you can deal with rework yourself for
failed boards then the cost burdon comes back to you.

Parts - jelly bean parts (normal caps and resistors
that is) they typically bought in reels by the
thousands and can reduce costs.  Special parts,
modules, chips, etc are usually (unless HUGE qty) not
purchased any cheaper than what you can but they have
full control over the buying process.  To elaborate,
if you have a part, its better to negotiate the price
with the vendor yourself then make sure that the CM
will buy it at that price.  We have seen where a part
has been nogotiated by a CM customer for lets say
$2ea, but our price is $2.25 for the same part. Why?
Even tho the CM is using thousands of this part, the
price was negotiated for the other client, not us. 
That was a surprise actually.

Best advice? If you can use a local CM its better
since you will have issues on first runs and also
during larger runs.  Visit the line, look at some of
the work they are doing, ask permission to talk to
other customers, get a good feel of the process.  If
they refuse to allow you to see the line, etc....might
think twice about using them (what are they hiding).  

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