I just went through this. My Viking is set into an Ikea table, and I love it. (Found an online tutorial on how to do it). But with the Juki, the machine I intended to do all further quilting on, DH bought a 3x6 butcherblock top at Lumber Liquidators and ordered adjustable legs from Ikea. Unfortunately, he didn't have the right saw to cut a perfectly-fitting hole through butcherblock (only a man would buy that heavy a top in the first place). So, since the Juki has a really nice and slippery extension table, I figured I'd leave it on top and try it.
This resulted in serious neck and shoulder pain. The solution was to raise both my chair and my feet, so that the extension table was at the height the table should have been. ( if this makes any sense).
So, IMO, sinking your machine into the table is best, but if your extension table is excellent, you can compensate.
The adjustable legs are a lifesaver and well worth the money. It gives you the freedom to change your mind about the table height AND if your floors aren't perfectly level, they solve the problem.
hope that helps a little.
hugs,
Charlotte