I had a DF500 and loved it, that is until I was bought the DF700XL as a present, now that is a machine.
I bought the Seneca adapter/shims which allow the use of the DF500 cutters on the DF700XL and decided to sell the DF500 (as a hobbyist I couldn't really justify owning both Dominos) and used the money recouped to by a Festool Tracksaw Outfit.
I have been building an MFTC over the past few weeks (bit here, bit there) and so far have used over 50x 4mm Dominos in its build, all cut beautifully and safely with the DF700XL.
The DF700XL is a heavier machine and can do mortices up to 70mm deep (depending on the cutter used), but has better ergonomics than the DF500, so I personally find it easier to use than the smaller, lighter model.
Both Domino Machines are great bits of kit and were an absolute game changer for me as I can produce excellent, fast superb fitting mortices where I need them and for the odd job, even produce my own hardwood Dominos.
The oversized 8mm Iroko Doinos shown below were specifically made to join the nine assorted 60-100mm x 1800mm x 24mm Iroko boards which I was putting together to make a Garden Gate for my next door neighbour, I used 5x in each joint (40 in total). I was advised to either make my own Dominos or purchase the Festool SIPO Dominos (silly expensive) as it was for external use. They were made to fit the wider slot of the DF700XL and to make them I just cut some Iroko 8mm x 24mm x 1800 lengths, ran them across a 3/16" round-over bit on router table on all four sides and cut them into 60mm long lengths using the SCMS.