I recently purchased an iMac 27" Core i5 unit intended to replace my 8-core 2.26 Ghz Mac Pro. I am selling the MP and with the pretty vast difference in prices, buying a drobo, some extra HD space, and using the rest for xmas gifts.

I primarily edited video and did heavy multitasking on the MP, but for the most part I use Safari, iLife suite,and occasionally Motion and compressor.

I found the vast majority of the time I was not coming anywhere close to saturating the 8-cores on my Mac Pro. But until the i5/i7 iMacs, there was no real alternative from Apple to a Mac Pro if you wanted the latest Intel chips or more than 2 cores. Now there is.

First of all, the screen is absolutely amazing. It's stunning, really and has so much detail it's almost too much. Next to my 24in LED Cinema Display, it makes that fantastic unit look lousy (?!?) in comparison (and is now next to it as a 2nd screen). The iMac's screen is whiter, about as bright (very very bright), and 1.35 million more pixels in a size that is about as tall as the 24" LED CD but wider at a full 16:9.

Make no mistake, this screen is so good with it's IPS panel and resolution and so forth it's almost worth buying to use just as a monitor (which it can). (27in iMacs MDP can be used as input) It's that good.

Bluetooth reception with Magic Mouse is flawless, unlike the Mac Pro. I don't like the bluetooth keyboard though; I stuck with my keypad-sporting apple keyboard from my Mac Pro.

This unit has a 1TB drive in it, which is pretty fast. However, I have a ton of video on multiple drives. I plan to move them all into a Drobo (I never backed the videos up, so this will finally offer me some safety with it's RAID protections) and use that via the 1 FW800 port. I know FW800 isn't as good as SATA. But I was surprised to see a bare 1.5TB drive in a USB 2.0 adapter run fast enough to (usually) play my raw AIC 1080P edited iMovie video with no issues. The FW800 Drobo should be at least 20-30% faster, so I am hoping editing video on it will be viable.

This is the one issue I have with the iMac that is most troubling - it needs more HD storage. (I keep hoping for eSATA ports on future Macs, including notebooks)

The 4850 graphics card is nearly as fast as my 4870 in general use. However, with the increased resolution of the screen and slower GDDR3 RAM, it can start to bog down if you're running lots of 3D apps at once. Or heavy games. That said, I did play XPlane and second life and a few others full screen and saw no slowdown or stuttering until I opened up 10 other apps like imovie and motion and ran them - and even then the slow down was minor.

In essence, the card performs better than the trusty 8800GT from Nvidia, not quite as fast as a 4870 radeon, but surprisingly close. Its a solidly quite fast chip for most purposes except for hardcore gaming under BootCamp. You can play there, but not at the screen's native res and get anything playable in most games. Never mind having 2 screens running.

Going from eight 2.2GHz Nehalem cores to four 2.6GHz non-hypertheaded cores seems like a loss. (Although most software doesn't use many cores) But surprisingly, in most general use cases the iMac i5 is faster. I suppose in single threaded apps, going from 2.2 Ghz in the MP to 2.6 that turbos more agressively to 3+ GHz in the iMac will show some improvement, and it does. The i5 iMac is simply faster.

I pushed this by launching 15 programs, including Parallels 5, iMovie, iWeb, mail, iTunes, Motion, Second Life, Photoshop, etc all at once, running through them in different spaces simultaneously. The machine remained smooth and fast, and the 4 cores showed maybe 80% use. This is a FAST machine, and only in certain special rendering situations that use all 8 cores (software I rarely used) will the 8-core be faster. Thus for most people, even some Mac professionals, the core i5/i7 machines will be plenty fast. I am impressed.

The only other practical limitation is expandability. Besides the HD issues I mentioned, I have 8GB now instead of the 12GB I had in the Mac Pro, and I can run less now before I see page outs in memory. I rarely filled 12GB on the Mac Pro however, so the 8GB tends to work nicely for what I do. But instead of having lots of RAM free, I tend to use most of it now in a given session. I wish 16GB didn't cost a thousand bucks. Otherwise, I'd have gotten it.

That said, 8GB can be added very cheaply using 2GB sticks compared to older iMacs, that only had 2 slots and necessitated expensive 4GB SO-DIMMS to get to 8GB.

So can the iMac replace an 8-core Nehalem Mac Pro? Yes. I don't feel like I lost any real speed (in most day-to-day, I gained in fact), 8GB RAM is more than enough most of the time, and the 4 Lynnfield i5 cores do very well. The i7 iMac is even faster - a real powerhouse - for only $200 more.

The new iMac is a machine of elegance and beauty and with the new quad-core models - practical speed for now and the future.
-Paul