I’ve used it, and it is wicked fast. You will get a huge speed bump, especially if you are coming from an Intel Core Duo (I am coming from an Intel Core 2 Duo).

Is yours a 24” or a 27” and what graphics card do you have? If you have a 27” with a Quad Core you’re still going to get quite a speed bump overall, but a giant leap in graphics performance for Aperture. If it’s a dual core, then it will just be a huge upgrade no matter what, especially since you’re going to get a SSD which will just make everything better.

Any which way, it will be great.

If it’s a 3 year old iMac, it’s a Core 2 Duo. That *2* is important as it means you can run Lion and Aperture 3.3.

The MBP Retina will be faster, but keep in mind that if you want the benefits of the Retina display, your Previews will need to be *much* larger. This could make your library significantly larger.

Assuming you have your original hard drive in your iMac, the SSD will be much faster on the MBP Retina and I’d guess more than make up for what otherwise would be a speed loss from dealing with the larger Previews.

If you’re close to an Apple Store, ask for some Aperture demos.

I have a quad-core MBP with 16GB of RAM and two hard drives (1TB HDD and 750GB hybrid HD/SSD). As impressed as I was with the Retina display, I’m passing on the MBP with Retina as I’d rather have the extra storage.

Also, like the iPad with Retina display (which I do have), I’m not convinced Apple isn’t pushing the envelop too hard in terms of what GPUs are reasonably capable of.