Alright, minor update here.
Compared to the latest iMac, mine is better specced than the entry level ones(2011 27" top of the line model), and I've been using it almost two months for editing and VFX. (Actually a lot of music "production" and other stuff as well, but that's irrelevant in this scenario).
Let's start with launch times. AE CS6 takes about 12 seconds to launch. That's on a 7200RPM hard drive. The new 21,5" iMac ships with a 5400RPM HDD, so expect it to take longer. It's not an extremely slow process, but it's tedious nonetheless, compared to 3 seconds on a fast SSD. When working with RED Scarlet footage, it's actually pretty snappy, but once you throw some actual effects into a 4K RAW comp, things start to move at Teague speed(pun related to road trip video). In general though, it behaves pretty identical to my i7 PC that clocked in at 3.4GHz, and this iMac sports an i5 running at 3.1GHz.
The biggest difference I'm noticing, is actually GPU related. My PC had an Nvidia card, utilizing CUDA and openGL, and even though the Radeon 6970M installed is pretty good for gaming, it's not really meant for video stuff, and leaves me wondering what Apple had in mind. Especially when the entire Adobe suite is geared towards Nvidia. Also taken into consideration that AMD owns ATI, and apple is running Intel CPU's in their computers. Oh well. It's not that bad.
Using Final Cut Pro X(hey, I like it, alright? latest update, 10.6, I believe, or 10.0.6, I'm not entirely sure), .R3D files run smoothly at 1/2 resolution, without stuttering. As is expected from FCP, it doesn't matter what effects or grading you throw on to the footage, the background rendering is snappy, and takes care of anything you throw at it without a hassle.
I've also tried using DSLR footage, to which it responds like a dream. No issues there, at least in the NLE bit of things.
So, in short; as an editing machine, it'll probably fulfil your needs perfectly. As a VFX computer, however, it falls short if you want to get things done quickly. If you get paid by the hour, you should expect to get more money, if your employers will accept the longer hours.
For VFX, I'd recommend(if you're going about it the apple way) a mac pro, maxed out. 32GB ram, 12-core Xeon CPU's, and SSD drives. At least for cache and system. Of course, that sets you back a million dollars, but so will an HP Z820 workstation. But also, the HP at least comes Nvidia Quadro cards, whereas a Power Ma- ehm, a Mac Pro ships with Radeons.
In the end, I guess it's all up to you, but for editing, the iMac will work like a charm.
Got any questions? Let me know what to test, and I'll give it a whirl. Keep in mind that if you're going for the entry-level 21,5", my results will be a tad better than that.