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The problem is that the design of laser printers inherently requires that data move onto the imaging drum at a precisely-controlled rate, and so laser (and LED) printers have traditionally included a CPU of moderate speed and enough RAM to image an entire page, either as a complete bitmap or as ASCII, using a quick font rasterizer to form images on-the-fly. GDI printers, however, offload this responsibility to the computer, and therefore require very specialized drivers that (a) are not available for any Unix, AFAIK; and (b) slow down the computer a lot when printing is underway.
Most GDI printers DO support HP's PCL in a lower resolution, so they can often be used from Unix via Ghostscript, but only in 300 dpi and/or missing some features. They may also be slower in this mode than in their native GDI modes. In order to print at 600 dpi with Ghostscript, a PCL printer must support HP's PCL at level 5e or better, so printer purchasers should look for that in any non-PostScript model, at least at the moment. (Naturally, all of this could change if/when support is added to Ghostscript for more esoteric models; but AFAIK, this is the current set of limitations.)
Finally, I've seen one extra twist on this already-twisted marketplace: The Brother HL-720 is advertised as supporting `PCL 5e for DOS' (or words to that effect). What this means is that it's a GDI model with a DOS driver that takes PCL 5e input and translates it to the printer's native GDI mode. Needless to say, this is useless for Unix.
GDI printers are a bad design even for the DOS lemmings, because they slow the machine down significantly while printing is going on. Like `WinModems', they're a sleazy way for manufacturers to save a few bucks. Our advice is, buy a printer with native Postscript and avoid all this crap.
Most popular printers are supported by GhostScript, and so it's easy to make them do PostScript. If you're buying any letter-quality printer (laser or ink-jet), check to see if it's on GhostScript's supported device list -- otherwise you'll have to pay a premium for Postscript capability! Postscript is still high-end in the MS-DOS market, but it's ubiquitous in the Unix world.
Warning, however: if you're using ghostscript on a non-Postscript printer, printspeed will be slow, especially with a serial printer. A bitmapped 600 dpi page has a lot of pixels on it. Further, if you're doing much printing, ghostscript will create enormous spool files. (megabytes/page). At today's prices, paying the $750 or so for Postscript capability makes sense.
If you're buying a printer for home, an inkjet is a good choice because it doesn't use gobs of power and you won't have the toner/ozone/noise/etc mess that you do with a laser. If all you want is plain-ASCII, dot-matrix is cheaper to buy and run.
Inkjets are great in that they're cheap, many of them do color, and there are many kinds which aren't PCL but are understood by Ghostscript anyway. If you print very infrequently (less than weekly, say), you should be careful to buy a printer whose print head gets replaced with every ink cartrige: infrequent use can lead to the drying of the ink, both in the ink cartrige and in the print head. The print heads you don't replace with the cartrige tend to cost nearly as much as the printer (~$200 for an Epson Stylus 800) once the warranty runs out (the third such repair, just after the warranty expired, totalled one informant's Stylus 800). Be careful, check print head replacement costs ahead of time, and run at least a cleaning cycle if you don't actually print anything in a given week. (Conversely, toner starts out dry, and ribbon ink won't evaporate for years...if you truly print only rarely, but neither a dot matrix nor a laser makes sense, consider buying no printer and taking your PostScript files to a copy shop...)
A parallel interface is a cheap way to make your printer print a lot faster than a serial line, and everyone's got a parallel port in their PC.
A few printers for the MS-DOS market require a special controller card and proprietary cable to do PostScript. These require MS-DOS software and typically won't run under Unix at all.
Meanwhile, there are several true 600 dpi lasers that grok PCL 5e, yet cost less than $500 retail. Currently (December 1997) these include the Lexmark Optra E (and E+), the HP 5L (and 5L, and probably 6L), and the Brother 760. As you can't easily buy a new hard drive smaller than 2 gigabytes, tens of megabytes of spare space in /var/spool should be the accepted norm, rather than a problem, for new systems; I've also noticed that PCL 5e seems to include some amount of compression (probably RLE or font encoding) which works rather well for text, further reducing the spool requirements.
One of our spies says good things about the Canon BJC-240 and 250. He reports they preint well with Ghostscript and are more reliable than Deskjets.
I personally have a LaserJet 6MP, and like it.