So we have used geofilters twice at our pub crawl events. It cost us no more than 6 quid and 12 quid respectively.
I think the most important thing is to make sure everyone knows its there! Our first event people didnt really know that it was there so as soon as guests arrive just tell them. We find that girls tend to use them more than guys.
Probably would give them another chance and make sure everyone knows but I personally do not know how many people actually used them. :s