I refer to the IFLR1000 especially with respect to jurisdictions I have no knowledge about. The main reasons: to find a good firm or confirm standing of a firm recommended by someone else.
I usually use the IFLR1000 in connection with a need to engage a local counsel on an M&A deal.
Corporate M&A lawyer
Listed international pharmaceutical company
We operate panels in many practice areas and in many countries. We don't always have a practice area specialist who knows firms in the country, so that is where guides like this come in handy.
The IFLR1000 is a useful cross reference to refer to ... to make sure a personal referral aligns with some objective rankings from a survey. I would refer to it in my main area, M&A, when looking for a counsel in a country we are new to.
The country-by-country tiered rankings by practice area are a good approach, to get a rough feel if a firm is viewed as active in a practice area.
UK General Counsel
US-listed multi-national corporate