Having been lucky enough to get a 737 sim ride last summer I took the advice to get some practice in, having never flown a jet before. I spent £1100 for a couple of hours to give me the best chance of passing, but with hindsight, believe that this backfired.
Of the three of us to be assessed on the day, there was a 3000 hour TP captain, a 1000 hour 737 FO, and me on 300 hours with 2 hours sim practice. When the assessors knew I had been in the sim the night before I was told I would go straight into the assessment with no demonstration on the first departure, as was the norm. So the 1000 hour 737 guy got a demonstration, but I didn't.
Having compared notes with a friend who sat the same assessment several weeks later, it is obvious the way our assessments were conducted were totally different. I got the impression that because I had had some practice, I was expected to "nail" it all straight away with very little guidance, but with only 2 hours in the sim this wasn't going to happen.
The other point is that the practice session, although good in many ways, had it's downsides. For example, the climb speed should have been 210 kts, and in the practice session I was told to raise the nose slightly to maintain it. On the actual assessment I was pulled up by the assessor for this and told to maintain pitch and allow the speed to creep up towards 230 kts (cruise speed). I was only told this after my first attempt. My friend was told before he started to ignore the 210 kts and go for 230.
This is just one example of the different styles I encountered, many of them only small differences, but nevertheless important. Things like at what point to begin a turn on the DME, etc. So for me, having practice helped me to get the feel for the handling with the pitch / power couple, but had the negative effects of me flying slightly different techniques to what was expected, and having had a "practice", being viewed as someone who was ready to go without any demonstration beforehand.
If I am lucky enough to get another assessment I will not be paying for any practice sessions beforehand.