Since I have a pair of these, I figured I'd add my two cents.
The NeXTstation Color Turbo, or the 'Color Turbo Slab', as it's known to NeXTophiles, is a surprisingly capable piece of hardware considering its age. The CPU, as many Mac and Amiga users are aware, is about equivalent to Intel's 80486DX2/66 MHz (or so, maybe a little slower for some things), and the framebuffer is not well accelerated (I believe it does have some acceleration, but not much). Nevertheless, the machine makes a capable web-surfing and email terminal, using Omni Group's solid OmniWeb browser, and NeXT's Mail.app (which, incidentally, survives to this day in the form of Mac OS X's Mail.app).
The Color Turbo Slab shipped with a 426MB hard disk, but these are really too small, even for a full developer installation of the OS. Fortunately, SCSI is SCSI, no matter what it was intended to be used in, so I've retrofitted my slabs with 4GB SCSI-II disks that I salvaged from a dead x86 box. Both are now happily running OPENSTEP 4.2. They also have ethernet, and can mount NFS shares from other machines that run UNIX, so both of mine also have access to nearly a terabyte of storage on my Linux box, should the need arise. SCSI also makes it possible to add an external CD-RW or CD-ROM drive, which the OS knows how to use.
These workstations make great geek toys, but can still do a surprising amount of real work, too, despite their rather glacial CPUs. A great many programs, both gratis and abandonware, are available for NeXTSTEP and OPENSTEP machines over the Internet, including the graphics program WetPaint, an old but functional version of WordPerfect, the browser OmniWeb, several X servers and lots of free UNIX utilities. As if this weren't enough, NeXTSTEP's desktop environment truly rocks.
So, you say, you want to find one of these keen machines? Well, eBay is a good place to start - I got both of mine there, for very little money. If you strike out there, check out the newsgroup comp.os.next.marketplace, various NeXT-related websites, or Black Hole Incorporated, which deals in NeXT gear. For a fully kitted-out Turbo Slab, including a 4GB disk, the full moby of RAM and a 17" color monitor, you can expect to pay anywhere from $100 to $550 (USD), though this varies according to supply and demand. Potentially, you can get one for almost nothing, as some don't know what they have other than an 'old, slow computer that works'.
The above writeup mentions the hyper-rare Nitro CPU upgrade - don't expect to ever find one unless one of the current owners dies. Instead, there is an accelerator known as the Pyro which can be installed, improving any NeXT machine with a 68040 33MHz CPU to 40MHz 25MHz 68040 to 50MHz. It's rare and expensive, but can be found. An enthusiast over at next.z80.org has also found a way to overclock a Turbo Color Slab to 50MHz. This configuration has proven fairly stable, and his hack also incorporates a switch that allows the hacker to select either 33MHz or 50Mhz prior to power-on.
The correction regarding the Pyro upgrade daughtercard was provided to me by many helpful NeXT wizards over at next.z80.org.