Going on 7 years with the 500 and I have no complaints other than the wider slots are too wide. The 2nd setting is a wide as I would ever want to go and so the 3rd setting is never used. Would like 3 options but scaled between the 1st and the 2nd. My Domino application has consisted of lots of 3/4" ply carcass construction, cabinet doors, trim work, table tops, room and exterior doors and furniture pieces including tables and beds. Except for the doors I very rarely venture into the 8x50 or 10 x 50 tenons. I do double up on 8X 40 sometimes before going to a longer length - depends of course on lots of variables
Having said that I did buy the systainer kit in the beginning and was happy to have done so. My 1st systainer came damaged and so I asked for a replacement box. I was told that Festool would not sell an empty systainer so I was sent another complete kit including the bits . I am now into my 3rd set of 5mm and 6 mm bits but expect that the 8 and 10s will outlive me. Just as my double collection of 8x 50 and 10 x 50 mm tenons. I've used up boxes and boxes and boxes of the 5x30 and 6x40 and 1/2 as many 8x40s. Just my experience. Which you might use will certainly depend on the kind of work you intend to do - just as Prashun has noted.
Yes, you do need good dust collection - so please don't compromise yourself out of that essential. The Domino is a wonderful tool.
Incidentally - I did discover not too long ago that the Domino makes an excellent elongated hole for attaching a stretcher to a table top with washer head screws. Bonus!
"... for when we become in heart completely poor, we at once are the treasurers & disbursers of enormous riches."
WQJudge