Even compared to modern standards, the iFly 737 packs features that make it a "hot" item for FSX users:
There is a massive library of free community-made liveries available online, covering almost every airline that ever flew the NG.
The Heads-Up Guidance System (HGS) is available for those tricky Category III landings in low visibility. Why Pilots Still Choose FSX and iFly
The pack includes the -600, -700, -800, and -900 variants, along with BBJ (Boeing Business Jet) versions.