I have 3 USB2 cases (2 x 3.5" and 1 x 2.5") and 2 USB2IDE cables and I have found the Vantec cases the BEST - the Vantec 3.5" case cost me 2500/- ... I think it is worth it to get this case. The other 3.5" case I have is the BAFO which does not recognise HDDs larger than 127GB.
Regarding speeds.....
when I have my 2 HDDs conntected to the computers Controller I get a data transfer speed of around 48mb/s
when I move one of these 2 HDDs to a USB2 case the data transfer speed comes down to 24mb/s
The USB2 2.5" case with a Seagate 20GB 5400RPM HDD gives me 11mb/s
This same Seagate 2.5" HDD when connected to the USB2IDE cable gives me 18mb/s
The Techcom USB2IDE cable is not good. The one to buy is...
http://www.dansdata.com/rdriver.htm
I got one of these from Lamington Road for 650/- 6 months back - I heard it is a bit cheaper now. The advantage of this cable is that it takes 2.5" and 3.5" HDDs as well as Optical drives and writers.