One problem with these types of stretching machines is that there is a common tendency to use them to "force" a split (which can often result in injury) and/or to hold the "split" position for far longer periods of time than is advisable. Holding a v stretch for far too long specifically on a stretching machine which will in fact make it feel like utter torture and that is not what your goal in stretching should be.

There is alot of disagreement about is how long you should hold a passive stretch such as the seated v-stretch.
Some say it should be held for as little as 10 seconds to as long as a full minute (or even several minutes).
Some researchers say 30-60 seconds... the more recent research on stetching the hamstrings indicates that 15 seconds may be sufficient enough time but even that may not be sufficient time for your other muscle groups.

It's important not to try to increase your flexibility too quickly by forcing the stretch. Stretch no further than the muscles will go without pain!!

I see nothing wrong with ocassional use to measure one's degree of flexibility. I used to use one after class when I was thouroughly warmed and stretched to measure how many degrees I was able to go.