I've got the Kenko DGX 300 Pro 1.4x and the original Canon 1.4x TC (in a battered state!).
Generally I find the Kenko has slightly more contrast, but shows more purple fringing than the Canon TC. You can still get the Kenko for £130.00 from DigitalRev, and it will fit nearly all, if not all, EF lenses. The Canon is less versatile in that respect.