I wouldn't take one if it were free.  Had one (a 2003 model bought
new)...dumped it promptly.  My advice, run like hell!

But to answer your specific questions.  Highway mileage is excellent (30+).
City driving mileage is poor (<20).

Yes there are some common problem areas -

- The A/C control panel craps out causing the system to cycle on and off
(mostly off when you need it to be on).  I disassembled mine, cleaned it and
it's working ok for now, but the replacement part is expensive if the free
fix doesn't work.
- The resistor that is integral to setting the fan speeds frequently craps
out, disabling the first 2 fan speeds with it.  The replacement part is $28,
is an improved design and takes 45 minutes to replace.
- Wheel bearings tend to go bad prematurely.  ~$150 each if you replace them
yourself.
-on my particular car, the Firestone Affinity tires have dry-rotted
prematurely.  The tires were nice and quite and have great grip in the snow,
but Firestone's impeccable reputation for quality is displaying itself right
before my eyes.

In general, I think that my 2000 Malibu is a decent car for the $$.  Between
$1K rebate, $2.5K of GM credit card points and some negotiating with the
dealer, I got a bought a fully loaded LS for ~$16K.  It was the best deal
out there - it's certainly not the best car out there, but for the $$ I'm
fully satisfied.