yes, most of mine are, Barrie Swaine saddles and my new Ideal saddles. the former are cream serge, the others are dark brown serge.
i never put it straight against the horse as it can be a tiny bit velcro-ish (doesn't slide as leather does) and some horses don't like the feeling. i don't clean it, other than brushing off the odd hair.
it is lovely and soft for the horse and easiest for saddlers to adjust the flocking of. highly recommended.
If you are going for serge just be careful as it absorbs the sweat much more easily and the flocking is prone to going more lumpy than a conventional leather. So you are likely to need to reflock more frequently, just something to bear in mind.
have had one for ages, with white serge, never needed to clean it but do use a thin numnah.
Can't say it goes lumpy or needs reflocking more often than leather saddles....
All my saddles have serge!!! (I have Saddle Company and Barrie Swaine saddles)
I do use a thin cotton numnah just to take up the worst of the sweat but I find the panels easy enough to keep clean. I tend to let them dry off and then put the hoover over them! If they are really thick with sweaty hair a damp rubber gloved hand picks it all up nicely.
i've not found that they require reflocking/adjusting of flock more often than leather lined. i always use a thinnish numnah and have never noticed the sweat affecting the serge - if it even gets that far.