Andy:
I’ve been to several. One thing you’ll notice is every ball has the same drilling. This will help you see the difference in ball reactions due to core and cover. Our local house puts out different oil patterns so you can see the problems that arise using the wrong ball on the condition. Normally we have two pair dry, two pair medium, two pair heavy. We also run two pair with the normal house shot in case someone is looking for a league ball.
The balls they bring in may not have a span that is exact for you. It may be hard to get the right feel out of the ball. IE, pitches, spans, etc. Usually you can put enough tape in though to get close.
Prices vary but usually run around $20 for the event. Ebonite gives good discounts if you purchase something during the event.
Since time is limited, do your research first and narrow your selections down. Depending on how many people show up and the length of the session, you may only get to try out 4-5 balls due to the wait getting the balls set up.
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~~~SrK - Have balls, will travel