The empire strikes back
This is aimed at on-the-fence Republicans who might be considering The Donald, not at the hard-core Trumpolines (who are probably unreachable at this point). But neither camp can claim they weren't warned.
Race42016 has comments by a range of rank-and-file Republicans. Horizons observes that Romney, despite his dignified delivery (a striking contrast with Trump) is perhaps not the best person to deliver this message. You Might Notice a Trend concurs, and offers suggestions for what the Republicans really should be doing if they want to stop Trump.
I tend to agree that while Romney's attack will give anti-Trump Republicans a rallying point, it won't slow Trump's march toward the nomination very much -- the forces Trump has unleashed can no longer be stopped. But it's a cornucopia of material for Hillary to use against him in the general, despite Romney's unfair digression against her in the speech. Never before has a party's presumptive Presidential nominee been attacked like this by one of the most important leaders of his own party.