I am a self confessed NT5 fan, but I did give Ubuntu a go earlier on this year.
Im a Linux admin by trade, but I have to admit that Ubuntu, whilst looking very nice and having made great strides over the Linux of old, is still very user-unfriendly and involves a very steep learning curve for anyone used to Windows.
The fact that they have still not managed to implement a basic point and click install system and instead continue to rely on repositories etc, and the OS itself is still hevily dependent on the command line, says to me that it will never become mass-market in its current guise.
Needless to say, after two weeks, I got bored with its limitations (it didnt run half my software) and fed up with it being so much alike the work I do and having to do everything the long way round and just went back to XP.
As for reliability, Linux is very reliable and stable, but I can't really say that I have any problems with any NT5 OS. 2000 Pro still remains the most reliable and solid Microsoft consumer OS to date, and I have only ever seen blue screens that are related to duff memory on my own systems in the past, XP since SP1 was released has caught up to 2000 Pro and is within touching distance of it, one awaits SP3 to see what improvements they can make, I think its major downfall is its somewhat bloated compared to 2000, but I have to admit I havent seen an XP system crash (on my laptop, media centre or mums PC) in well over a year, probably more like 2 years except for when I had a faulty hard drive in my laptop which was causing it to randomly die and reset.
I keep an eye on all my systems, and perform maintenance maybe once a month on my laptop (which is used every day, probably for around 12 hours a day on average) and once every 6 months on me less-used media centre. Both have not shown any significant signs of slowdown, and still run like the day the OS was installed.
My mums PC was a different story, because it was crucified by my sister with virus's, "free-search" software, numerous other pieces of software running in the background and was bog slow. After spending maybe 25 minutes cleaning up the mess she had left, it runs like a dream once again.
The OS on my laptop is only around 4 months old due to a hard drive change, but the OS on my media centre is from 2003 iirc, and the OS on my mums PC is from 2004. So the fact that these are still running fine with no slowdown, goes to prove that its how a user looks after a system rather then the system itself. The media centre is only a Pentium III 850Mhz with 384MB RAM etc, and it handles everything bar gaming, even CS3, just fine, with Windows having been tweaked so much by myself that it uses less then 80MB of RAM at idle on startup.
If the user just installs anything and everything, and expects it to keep running without any realy sort of maintenance, then its like doing that to a car, it will eventually lose performance, lose efficiency and get bogged down, and probably actually fail. If its looked after and maintained to a basic standard however, then it should run pretty fine.
I think its wrong to say Windows is slower then Linux, the only place where Linux excels over Windows is memory handling and allocation, but Linux is a server OS come desktop OS, and is very unusable as a main desktop OS imho, wheres Windows has always been designed as a user OS.
Microsoft never moved too far away from the original concept of Windows being ultra user-friendly and throughout the NT line it very much mirrored the consumer line in terms of interface and usability. When 2000 Pro was released, it was clear the direction Microsoft was taking was ultimatly going to lead to an NT based consumer OS, and Windows XP was that OS.
NT6/Windows Vista however, is a different story altogether, but the less said about the better. The point is, NT5 is near-perfect in every way, not just certain ways, which Linux can boast. OSX with itself having its roots embedded in Unix, is probably the only consumer OS that can take the title of the perfect OS, but its limitations as to what hardware it will run on unfortunatly both price it out of the market for many consumers and will continue to ensure it only has a niche market.
May I also add that my mums PC has not actually got SP1 running on it yet!!