I have a new imac.
Before I had a Sony Artisan. So I'm used to just putting puck on, walking out of the room, coming back and it says the monitor is calibrated. Everything else is just over my head, and honestly I like it to be like that.
If this is what you're after as mentioned you could get a screen that has hardware calibration (NEC xx90 series (there's a few others as well in the NEC lineup), LaCie, Eizo, and the HP Dream Color, etc.
Now with regards to the iMac you can actually get this. http://www.integrated-color.com
On the iMacs, MacBooks (and MBPs) as well as all Apple Cinema Displays, Color Eyes Display Pro can take control of your backlight. Profiling apps like Eye One Match and Datacolor don't have this capability and you have to adjust the backlight level manually before it profiles the screen. While what Color Eyes is doing is not exactly the same as what was going on with your Artisan (the Artisan was being calibrated by software, that is the software was directly adjusting the RGB levels on the screen itself which is a true calibration vs. an iMac which aside from the backlight level can only be profiled) for all intents and purposes the end result will be the same (with regards to the fact that you press a button, come back, and your screen will be profiled).
With regards to the puck... I have a small collection. I have an original i1, an i1v2 (which is supposed to be optimized for wide gamut displays, the i1v2 itself is not otherwise well suited for use with wide gamut displays), a DTP94, and a Spyder 3.
One of the other readers here suggests that there's some QC issues from sample to sample with the Spyder which is very possible if they're using lower grade IC components that don't have tight tolerances or precision (which makes them less expensive to manufacturing thus yielding a higher profit margin). I think I got a good copy. I'd say mine is ever so slightly better than my DTP94. When used with wide gamut displays (and the 27" iMac qualifies there) the Spyder 3 produces more consistent results than the DTP 94 which really kind of falls down a bit with wide gamut displays (especially with LED backlighting which the 27" iMac uses).
One thing you do want to do with any puck is to leave them plugged in for at least 10 minutes before you profile your screen to give the puck some time for the electronics to heat up to operating temps and stabilize (same goes for your screen although with your screen I'd recommend 45 minutes to an hour).
With regards to the iMac screen... Historically all of them have been pretty terrible however Apple may have turned over a new leaf with this one. In the past one of the biggest problems with the iMacs was that they could not be calibrated to a luminance of 110 cd/m^2, they were just WAY too bright. This new machine though can although I don't know if that's purely through a backlight adjustment or if Apple is using software to compensate via tweaking the video LUT to further bring the level down after the backlight has bottomed out... if that's the case that's not really ideal). The downside with the iMacs of course is still the glossy screen. You really need to control your environment if you're going to be using one of those screens.
If you're a full time professional I'd strongly recommend getting a decent monitor as it's the single most used piece of gear in your studio (well, one could argue that your chair gets used just as much :-) and having a really accurate screen can save you gobs of time if you ever run into some tricky issues. If you're an enthusiast then I'd say the screen is an optional thing, you can certainly get that iMac screen to work for you, it's more a question of budget and priorities.