I gave it a go about 3 years ago and gave up after 6 weeks. From memory you have to use it everyday for about 20 minutes, it requires commitment. Can't say I noticed any difference.
Slendertone also do a facial toner which purports to be the best if you believe the advertizing. It is also more expensive than Cleo.
I am currently using Rosemary Conley Ultra Facial Flex which is supposed to firm up the face muscles, have been using that for about 2 months, can't say I notice any difference with that either but I am still persevering. Costs about £25 and requires 2 minutes of your time a day.
I have come to the conclusion if it's the skin which is getting saggy as in my case, around the mouth, then no amount of exercise/stimulation is going to do the trick. I am going to see a friend of mine who runs a cosmetic surgery practice in Harley Street to see what the solution is. She swears by Botox and fillers but don't want go down that route.
If I just slightly push the skin back to my ears that's the look I am after. Probably means a lower face lift.
I try not to look down and keep my head slightly tilted upward if I can to stop the skin sagging - not easy.
I wish I could grow old gracefully - but it ain't happening.