I've owned the original and the Plus model, and played extensively with the QS8. As I'm sure you know, they are fairly basic romplers--filled with (mostly) excellent samples--the piano in all models except the first (which sucked) is one of my favorite sampled pianos--but not a whole lot else to recommend them. To answer your question, pads are another strength of these boards, if you're into complex, expansive, digital-sounding pads; it sounds to me like Alesis was after a D-50 vibe.
Seems like they'd make a perfectly competent but not outstanding master controller, depending on what you'd use it for.
All in all--
-Plenty of good samples (apart from the original's crappy acoustic piano)
-Nice keyboard feel. The original's is my favorite, and quite good at that; the Plus' is a little stiff for my taste. The QS8 has a very respectable weighted action which is quite nice for pianos, but I and my friend who owns one find it kind of awkward for other types sounds. Midi to the rescue...
-Nice, comfortable rubber controllers (although I prefer the later models' sliders to the rubber knobs on earlier models).
-Good effects--they included more or less a complete Quadraverb processor.
-Fair build quality. Older ones are plastic--albeit sturdy--newer ones are metal (and quite heavy). I can attest to my friend's QS8 holding up after being taken apart and generally abused by some overzealous airport security personnel.
-Pathetic filter. It's lowpass only, very shallow (I'd guess 1-pole, maybe
2-pole), and lacking resonance. There are a bunch of resonant waveforms, but that really doesn't quite cut it.
-Weird patch organization. This has been dressed up in newer models, but I think it's the same scheme.
-Older models have nice, big LED screens (which are nevertheless cramped); Alesis manufactured these specially for these boards and they are impossible
to find if the backlight happens to blow, as mine has. Newer models have a tiny (but reliably backlit) pixel-based LCD screen.
-As with so many romplers, editing is a complicated, dissatisfying chore which is exacerbated tenfold by a non-backlit screen.
-The stylish buttons on the early models are susceptible to dust, rendering them a little unresponsive.
-They lack a lot of standard 'workstation' appointments (not that they are meant to be workstations)--sampling, sequencer, arpeggiator, and so forth. I actually prefer it that way, but that's just me.
-One bank of overwritable user presets (each, for individual patches and multitimbral setups) compared with 5 banks each of factory presets in ROM (they can be edited, but must be saved to the user bank), plus a GM bank.
Hope that helps.