I only ever went to one walk up, the long gone Les Girls, which was perhaps the most famous Soho walk up. This was in the days when I was more adventurous. I recall she was English which would be unlikely now, but a bit of a fatty and I didn't rally enjoy it.
Les Girls was more of a standard parlour than a walk up, they certainly had a much higher standard of girls there and the minimum appointment time was 30 minutes, at least during the time I went there, from about 2001 until it closed. They had some great girls too, like Carla and Kirsty, where french kissing was offered (a service which can't be found in most walk ups).
These days I don't like 10 - 20 minutes appointments, and only punt in Soho if I'm really broke, but prior to the internet and when I was a young 20something I punted there a lot - and have some fond memories of seeing one girl for a quick shag, before seeing another for a 20 minute session half an hour later, all for a total of £50 - £60.
I remember one time a girl told me how she'd just seen this bloke who'd paid £20 for a blow job, but stopped her just before he was about to cum. Apparently he'd done the same thing with three other girls that day, and had one last punt planned after her. She found it quite funny, but I imagine there are worse ways to spend a day. As long as that last girl gives decent head, at least!
It's definitely gone down hill of late, back in the early 2000s there were quite a few gems working there, but these days every so often one pops up, inevitably burns out after a few months, and then you have to wait a while before the next one does. Nonetheless, I'm glad it exists, if only because I got to shag the infamous Paris for £30!