If your hardware is supported, haiku is fantastically fast on even extremely old pentium hardware. I tried it on an ancient celeron with 256m ram and it was amazing even running off of a USB 1.1 drive. The mail app starts in less than a second. The browser starts in about 2 seconds and feels as responsive as safari on my i5 macbook pro with gobs of ram.

The coolest feature, however, is how well it preforms under stress. There is a 3d demo that maxes processing resources and displays the performance in FPS. As soon as the demo starts, the CPU is pegged at 100%. Even with the demo running, the system is still fast and stable. Haiku, like BeOS, won't let one process slow the whole system. Apps still load in roughly the same times and remain responsive. I haven't tested how well it handles memory hogs, but I've read that its nearly as good there too.

As long as you are only using apps that come with the distribution you might could use it as day to day desktop, but I think it would probably be a diservice to the users and the project to recommend Haiku in its current state to anyone other than tinkerers and devs. Finding apps would be a problem, even if you didn't have to worry about whether or not they would run. A bad app will crash the whole system, although stability has come a long way in the last couple of years. You just can't forget that this is alpha software.

I think things will rapidly improve though. There is a package manager on the way which will be finding software easier. Also, I've been reading about some fantastic advances in programming language support which should increase the amount of software made available.

Edit: I'm trying to walk the line here, I'm clearly a fanboy.

Haiku hardware support is patchy. I doubt anything would have better coverage than linux on shitty hardware.

I find UI to be the driving force - you get a GUI that interacts with the filesystem much more directly than you can in any other GUI.

A problem I've found for workstation usage - the terminal has some problems. For example, I can't connect over ssh to a remote linux system and have GNU screen run there usably. There's a problem with the way colours are implemented. I understand they're workign on a tty rewrite before the beta but am unsure what the scope of that is, will it deal with this.

I received a haiku programming book from amazon just today. I'm interested in exploring this and in digging towards the OS API - seeing what they do differently to unix. Maybe I'll find cool mechanisms for prototyping that aren't available in linux but I'm skeptical there'll be much.