Compared to the scores I was getting using the recently retired tests, my actual scores varied as follows on the April '07 MCAT:
1 pt higher than my best score on Verbal
1 pt lower than my worst score on PS
Same as what I consistly scored on Bio
...so in my experience, it was a fairly accurate measurement of what I was likely to score. If it was me, I'd walk in assuming you'll get one point lower than your lowest score on each section, and if that's a score you are okay with, go for it. That way if it's slightly higher you'll be pleasantly surprised.