Here is the projector heirarchy (beginnng with the Best and ending with the Worst) by manufacturer....

1. ELMO - The Cadillac of 8mm regardless of what model.  There isn't enough one can say in praise of this manufacturer.  These are often very pricey, but well worth it.

2. BELL & HOWELL - Very nice machines and usually very reliable and stable.  Easy on film.

3. EUMIG - Solid construction is a norm for the EUMIGs, but they can be tricky to thread and they have been known to scratch a film or two.  Also, the sound heads wear out pretty quickly so beware if you get one - chances are there aren't alot of hours left for sound projection.  Don't pay alot.  EUMIG (to the best of my knowledge) is the only maker to have created the DUAL 8 SOUND models.  These machines will play both Standard and Super 8 Magnetic Sound film with the simple switching out of a sprocket device to allow normal playback.  Bid on one with minimal hours use.

4. SANYO - Nice machines, overall.  The electrical circuitry and motors tend to crap out fairly quickly so again don't overbid.  They are gentle on film, though.  Try to get one with little-to-no use.

5. CHINON - Nice machines.  They are temperamental, though. The sprocket drives and film claws aren't always the most reliable, but they are gentle on film.  Try to get one with very minimal use (ie: "...Rarely used, still in original packing materials when originally purchased", etc.).  The belts on the CHINONs are of a springy aluminum material and are extremely difficult to replace so when it craps out on you, simply throw it away or sell it as a parts machine.

6. GAF - Nice machines, but the bulbs might be a little tricky to get for the DUAL models (they usually take a DMD or DGB).  The belts are easily replaceable, too.  However, the DUAL models will often eat film so make sure your precious prints have plenty of leader before the actual film starts.  I tend to like these the least.  I've had too many prints chewed while threading.  The sound units were actually made by Chinon and ran somewhat better.  It's the DUAL machines that gave me the most trouble.  Don't spend alot, whatever you do ($50 bucks, tops).

7. KODAK - Junk - Pure junk.  Kodak made awfully nice 16mm machines , but they're 8mm and Super-8 models are pure unadulterated crap.  Please... Don't even bother looking.

8. KEYSTONE - Nice machines.  A little tricky to thread, but very solid construction and very reliable projection on a consistent basis.  Warning - Bulbs will be hard to replace and the circuitry is often obsolete (tubes and such that will hum when the unit is powered on and sometimes give off a burning smell).  Be very careful that the unit nor your outlets overheat.  Make sure there's no spliced wires in the back of the unit or copper pennies cut in half and inserted somwhere as a conductor of some kind.  Ask alot of questions before bidding and ask to see a picture of the back panel (CLOSE UP).  As long as all else checks out ok, these units are terrific machines and will run forever.  Again, you want to see pictures and ask alot of questions about who previously owned it and was it ever serviced ( if it has been serviced - BY WHOM?!).