Performance is relative and depends on what you want to do. The UNIX and PC worlds are now looking at implementing the same type of IO structure (channels) as mainframes have used for years. This is because of the IO capacity the mainframe has. The just announce zSystem has increased this capacity even more. The new zSystem can support 24 Gigabit Ethernet connections or 36 100 MB FICON connections at wire speed. There are very few UNIX or Intel boxes that can even get 24 Gigabit Ethernet connections and those that can normally only have 4-8 independent IO busses and therefor every 3 - 6 cards are sharing the same IO bus and the buses are only running at just over 1 Gigabit. If you have 3 Gigabit cards sharing the same gigabit bus, you are not going to be able to drive all 3 cards to a gigabit. I have not seen any test recently, but about a year ago the fastest Intel box could only get about 800 mbps.
 
There was a company in northern Virginia that took a G5 box, created a LPAR with two processors and 256 MB memory. They ran VM in that LPAR and then ran Linux under VM. They ran Apache Web Server on the Linux systems. They kept starting more and more Linux system until they could not do anything else. They got 40,000 (yes 40,000) Linux systems up and running on a G5 2 way with 256 MB memory. Now is it practical to run 40,000 images, well no because this is where the system stopped. However IBM does state that you can run thousands of production Linux systems. In fact in one of their ads for the new zSystem they quote a price of $500 per Linux image, this is based on 2,500 images. This cost only includes the zSystem (CPU, Memory and IO cards), does not include DASD or any other external equipment.
 
When you look at the price you need to look at a lot of thing. Number of users, amount of work done by box, number of support personal per user. A single NT box is cheaper that a single mainframe, however you may need a lot of NT boxes to do the same amount of work as a single mainframe. IBM and Compaq has recent set some records with their Intel servers running Windows NT. I believe the Compaq solution cost about $12 million and the IBM cost $14 million. Not so cheap.
 
Now is the mainframe right for everybody, no. UNIX and NT have their places. Heck, even Microsoft knows that NT can do it all. They use Unix for some of their internal systems and for some of their services. Microsoft runs Hot Mail, Hot Mail runs on Unix, not NT.
 
The other thing that mainframe have had traditionally is hardware and software reliability. As Intel and UNIX boxes have some of the same hardware monitoring functions built into them, they are becoming more and more expensive.