A note with expert flyer that there is variation in availability from airlines in different buckets depending on point of sale. Not so much of an issue when flights are wide open, but as flights become fuller the correlation with airline availability is not perfect when using a different point of sale.
For example i find that SQ availability for flights bought in Singapore or Australia on SQ website are often different to what is seen on Exper Flyer, when selecting US as point of sale, but much more strongly correlated when selecting UK as point of sale.
But generally it is pretty good, and we too started with the free service with a few queries a month and soon found it worthwhile to take out an annual subscription.