The domina studio I attend occasionaly has a Venus 2000. This is a fine piece of equipment.
Especially if you like to be tied up and 'forced' to cum. While calming and comforting on slow speed, you (or at least me) just can't resist if it is on medium speed.
At high speed it tickles like hell - especially short after an orgasm... And sure, my mistress knew this very well...
Needless to say, it doesn't matter what else she does to me in a session, but I like me my to be bound to the gyn-chair, Venus on my member and to be 'forgotten' for half an hour (it really brings this 'merciless exposed to the evil machine'-feeling)...
I experimented with a cow milker for a while - bought some used bucket-milker from ebay. The teat-cups are small, but it works for me, if you cut out the rubber at the opening (me having a somewhat average size).
I also don't use some industrial / agricultural vacuum pump to operate the pulsator, but some smaller sized item. But one thing: You need a whole lot of lube to make it somewhat comfortable.
Besides this, the suction of the machine is a really great feeling and different from the stroking effect of the Venus.
Later I bought a SPT (standard, with electrified liner) from SeriousKit. This item works way better than a teat-cup, the material is smoother and you have a seperate hose just for lube.
Anyways - I use it sometimes and I had some great orgasms with the milking machine.
The two main problems with milking machines in my opinion are the noise and the work cleaning it afterwards.
It's really loud (way louder than the Venus 2000) and cleaning it is a hassle.
I still am very into watching men (and women) getting hooked up to milking machines of all sorts and I still use it occasionally. It is still one of my favorite fantasies.
Without those problems I probably would be hooked up more frequently...