Yep, I think G4 Towers are often seen as 'Boring' PowerPCs, so they're one of the few macs of this era that can be found at a low price without too much searching. I think they're cool though, with all the easy upgrades available you can turn a higher end one into a decent workhorse.
I love them-in fact I love them enough that I quit counting how many I had a long time ago

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The Quicksilver remains one of my favorite case designs, while the Sawtooth seems to have nearly bullet proof reliability. I have a soft spot for the DA, as my first G4 tower and it has the nice combination of the graphite case with what is more or less a Quicksilver LoBo(along with the faster bus speed and great number of PCI slots that comes with that). I actually went out of my way to get DAs in every factory speed, plus have several upgraded ones-including a dual 1.8ghz Sonnet and a 2ghz Newertech 7448.
I've not gone as wild over MDDs, as I had some early bad experience with the PSUs. Still, though, I love the built-in expandability, the GPU compatibility(there are cards that for, inexplicable reasons, will work fine in an MDD but not reliably in a Quicksilver or DA), and the fast bus on the higher end models. The 1.25ghz models, whether single or dual, are OS 9 beasts. The Dual 1.42 is competitive-both in benchmarks and in real world use-with faster clocked upgrades in earlier towers thanks to its 2mb of L3 per processor backed by a 167mhz bus.
I do a lot of photo work, both film and digital, and finally retired my last regularly used SCSI based scanner not too long ago. SCSI in a G5 is a nightmare, while it just works in a G4 provided that you're not adamant about using Leopard. In fact, within the past month I've used my "backup" Nikon Coolscan III, which a superb scanner that sells for a little of nothing because it's SCSI. I run it in OS 9 on the dual 1.8 DA...and right off the Adaptec 2930CU that shipped from the factory in a lot of G3 and G4 towers. The Coolscan V I usually use has a lot of advantages(aside from being USB, it is higher resolution and perhaps most importantly has more dynamic range) but the III can still turn out a perfectly acceptable scan. When the new Ektachrome started getting in end user's hands last month, I was one of the first people I know of to post scans, and all of my initial scans were done on the III.