Hi,
      It depends on which ones.  The Superformance Cobras from South Africa are considered the best - 
www.superformance.com, but I also like the all-alloy ones from - 
www.KirkhamMotorsports.com  These are produced by a former MIG 15, 17 and 19 Licensee in Poland.  Of course, Shelby is also making a 
replica - 
www.shelbyamerican.com which is probably quite good too.  The main difference is that none of these are using actual Bristol Ace chassis.  IMHO, all the others are ********************.
      For the Speedsters, I think only the Beck - 
www.beckspeedster.com  is worthwhile.  I also like their 550 replica.  I have a friend who has an original 550 (#47) from new (see pic below - he and I discussing the car) , 
and a Beck 550, which he drives because the original is too valuable to risk having some immigrant ram into it.  He says the Beck is better than the original - more power, better brakes, stiffer chassis. I drove his original, but not his Beck.  I did drive one Speedster replica from Boulder Speedster - 
www.boulderspeedster.com and wasn't impressed.  Poor Fibreglass lay-up and felt like a kit car.
      Hope this helps...
Happy Motoring!... Jim'99