I have both. If you can afford both, I think that's the way to go -- I like having a dedicated desktop machine on my desk, and I don't like hooking up tons of wires each time and running through startup to have a quasi-desktop.
That given, I did sell my Mac Pro, I gave my mom my iMac (I still use it sometimes), so my MAIN machine here now is my MacBook Pro hooked to a 23" Cinema Display.
I have 4GB of RAM on the MBP and run my display at a nice 1920x1200 with the nVidia 8600 built-in card, BUT there are still some drawbacks.
My MacBook Pro sports a tiny Mac Mini sized 160GB 2.5" 5400rpm hard drive. This thing can really choke, and a fullsized 500GB or 750GB 7200rpm HD, or even an SSD when they are available would be much better. The hard drive on the MacBook Pro is the one thing I don't like. Laptop drives will still be slower and smaller unless you have a screamer-SSD, even given that -- size will still be smaller (128GB or 256GB).
It does get hot sometimes, if you are running a render or something really processor-intensive. An iMac, even, would handle this much better.
Another thing, I burned my battery up (it shows an "X" now and will not charge), because I left my MacBook Pro on sleep, while I was on vacation for 3 weeks. The machine woke from sleep and crashed, and depleted my battery, really messed it up. Now I have to get a $150 new battery for the unit.
You don't have much power, so if you want to hook up more than 3 USB devices you need external Power. The iMac supplies this power already.
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so...
Those are basically my issues and holdups using a MacBook Pro as a desktop replacement. I will say it does work pretty well, and the performance is not bad. It still isn't as fast as a true Desktop such as an iMac would be...the iMacs are fast, and snappy, do not doubt their power. The Real desktop size/speed hard drive makes a huge leap in speed performance on the unit versus a laptop drive.
Also, I have been extremely happy with this 2.4GHz 15" Santa Rosa MacBook Pro, and think it is the best laptop I have ever owned (I've owned about 30 PowerBooks over the past 15 years). It's really Powerful and the screen is great, it's bright and crisp. Everything is snappy on the machine, but an iMac is still faster for some things. I like that power on my desk at home and would like to get an iMac soon, or possibly an SSD Mac Mini if they do come out. More likely the iMac.