Depends on what you mean by "underpowered." Yes, the iMacs are Core 2 Duos, but not too many people have the need for dual quad-core processors such as you can get on the Mac Pro... but I really like the really fast speed of the of the 1066MHZ FSB with DDR3 memory.
I have clients running their offices using the latest iMacs and are having no problems. That includes rendering hi-definition 3D x-ray images from a Cone Beam CAT scanner, etc. QC issues are actually almost non-existent with Macs. What you are seeing at Macrumors always claiming such are generally the postings of non-Mac users. Overpriced? I paid $1199 for my 24" iMac from the Refurb store at Apple.
You can have the best of both worlds, if you like. I have replaced my Windows laptop with an Apple laptop running a windows virtual machine via parallels and it’s great.
All you do is buy an Apple and a software package called Parallels. Install Parallels on the Apple and it will allow you to install a Windows machine on the Apple, too. It works well.
The other thing that people need to remember about the performance of Macs is since Apple sells full computer packages, most Mac software is written with the Mac models in mind so things generally work well for common Mac configurations for several years.