Hi Sole traveller, welcome to the forum :)
I have travelled with both JY and Travelsphere (JY's sister company). In my experience the trips have always been very well organised and really enjoyable.
I'm going on the 'Walking in the Ahr Valley' trip in June and will probably go on the Prague trip later in the year.
I've met some really interesting people on the trips I've done and had a really good time so I'd thoroughly recommend giving JY a go.
If you have any specific questions just ask - I'm sure one of the forum menbers will be able to help.
Julian