One word of warning about the MKIV. The "tang" of metal connecting the head swivel to the halyard has shown a tendancy to fail. As in, mine did fail halfway through the Hook race. Harken responded immediately and sent a new (re-designed) swivel assembly at no charge. $43 to pay the yard to retrieve my halyard, along with the shackle and broken tab of extruded aluminum that formerly connected to the swivel car that came down rather unexpectedly with the sail. That, and two hours of personal labor and problem solved.
According to Jim at Harken, the Unit 1 and Unit 2 MKIVs have had the problem, apparently due to an underdesign in the interests of limited weight aloft. I bought mine in March 2008; no clue if/when all the units on the market have the older design versus the new one. I just installed the re-designed unit last Saturday, so far, so good.
While I was disappointed that the failure occured, I will say that Harken stood up immediately and took responsibility for the issue without even a hint of denying there was a problem. So yes, I'd buy the unit again, but I would find out which swivel car I was getting...