I promised to report back on my experiences after having given this matter a more thorough test. This is on a Series 5 TV model bought in the UK.
I have been using a Cruzer USB 2.0 memory stick and have found it to be consistently reliable on both instant-record and pre-set forward planning (when the TV can be put into standby, I was pleased to note).
As for memory space, the original 4GB stick holds 90 minutes of standard-definition recording, which is a bit limiting, so I began using a 16GB Cruzer instead. Oddly enough, the price was about the same.
When this stick was initially formatted by the TV, it was stated to be below the minimum necessary speeds and unsuitable for use. So I turned off the TV, removed the stick, switched it back on and replaced the stick. Then I carried out a speed test (under "Tools") and, from then on, it has been recording perfectly - but I haven't tried High Definition which would presumably need the higher speeds.
I agree with what has been said about an HDD being better in the long run, and obviously its higher storage capacity would be great. But all I want is something which will temporarily record the odd programme or movie with the minimum of fuss, and I find the memory stick just ideal for that.
Selecting a stick with fairly high read and write speeds is essential, and I found this website invaluable as it lists actual test data by many different users: www.usbspeed.nirsoft.net/
Also note that USB 3.0 is definitely not compatible - some dealers substitute a 3.0 upgrade at no extra cost, thinking they are giving you a better deal!