Maybe someone can contribute with their own experience?
Mine is quite stable (more stable than my drawing table with L legs) up to 90-95cm, you can do delicate work without a problem when sitting. It gets a lot more unstable over 100cm, but you can still write on it until approx. 120cm. I have mine at 125cm when i am standing, at this level every button that i press on my gear causes a noticeable wobble (maybe 2-3mm, definitely too much for smd work). It is rather nice when you are working on equipment that is too large/bulky to repair when sitting (big oscilloscopes/AVRs, etc) but too low to work on comfortably when you are standing.
Another thing to look out for is the max. load, most tables are rated for 60-70kg (not a lot if you have equipment on it). Mine is rated for 120kg, but judging from the sound of the motors at 70kg load i wouldn't recommend going over 80-90kg.
If you want to test such a desk in person, i'd recommend a trip to Ikea: Their electric desk is a bit on the wobbly side, but it should give you a fair impression of the stability of electric desks.