The Lite version of Daemon Tools is a great little freebie for mounting ISO and other images as virtual CD/DVD drives, but the Pro Advanced and Pro Standard versions offer considerably more functionality. If you deal with CD/DVD/Blu-ray images on a regular basis, they just might be worth the 25 or 40 euros they'll set you back.
The first thing you'll notice about either Pro version is that there's a straightforward, easy-to-use GUI--no command line visits for advanced features as there are with Lite. Both Pro Standard and Advanced also support more virtual devices: 16 and 32 drives respectively, and Advanced offers 2 virtual IDE devices in addition to the Lite and Standard version's SCSI devices. Both Pro versions also let you mount the contents of an optical disc to an NTFS folder (instead of a drive) as well as letting you edit images (add, delete files, etc.).
Both the Standard and Advanced versions of Daemon Tools Pro offer cataloging and creation of image files, but Advanced takes things one step further with conversion between various image types. All versions support a number of file formats: plain and compressed ISO files and media descriptor (MDS), for starters, as well as the formats belonging to Blindwrite, Blindread, Nero, CloneCD, DiskJuggler, and InstantCD/DVD images.
The only real negative to Daemon Tools Pro is that neither the Advanced nor the Standard versions (nor the free Lite, of course) handle copy-protected discs. However, if you don't need to virtually mount commercial games or DVD movies, then one of the two versions of DT Pro should provide all the capabilities you need. With its improved virtual device support and its image-type conversions, Daemon Tools Pro Advanced is better suited for heavy or specialized use.
Note: DT Pro Standard and Advanced are included in the same download. You may choose which the one you wish to try at install time via a drop-down menu.