I've owned the Delta for about 10 years and it performs well. I hear quite a few new owners complain that the machine isn't operating correctly or is burning the stock. It usually comes back down to buying the cheapest chisels they could find.
If the budget allows, get some Clico chisels. You'll pay more but they are worth it.
I have the add on kit that you use with your drill press. I do very little mortising. Usually when making furniture. It is a Delta kit. My buddy has the dedicated Delta mortiser and he enjoys it. I used an oscillating chisel mortiser (MAKA) when I was in a shop a long time ago. Man, that is a dream machine. And for only $15,000 you can have one if you can find one.
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