Well first let me point out that when using Ardour you are actually using two things, one is Jack which is a low latency audio subsystem, the other is Ardour.
Jack is a VERY cool program, the closest equivalent on Windows is ReWire, however Jack I find much more powerful and flexible myself. It allows me to route audio between programs as well as between outputs and inputs on my soundcard(s) and any combination thereof, so long as that program supports Jack. On OSX this is any program that uses CoreAudio. On Linux you can get almost anything into Jack, but sometimes it takes a bit more work for older program, or programs especially corporate written ones that stupidly decide to use OSS for their audio. But most audio programs intended for pro use on Linux use Jack. Its the consumer programs that tend to be the problem.
I use Ardour on two machines, one is my main workstation that I dont have out here with me on this gig. That is a straight linux box set up for two things, audio and basic 3d modeling. Primarily audio though.
By low latency, lemme try to explain this...
Latency, in this respect, refers to the difference in time between audio being played back and audio being recorded. It mainly comes into play when you are tracking, or recording one or a few tracks at a time, and layering the rest of the tracks on top in other sessions. This can show up in two ways, one the audio being recorded being slightly behind the audio being played off the computer, or two, is the audio recorded is monitored as it is recorded, resulting in it being heard slightly behind when it is performed.
Most true audio sequencers are set up like this, Audition 1.5 and earlier were not capable of it really, but with the update to 2.0 it may be, again I havent used 2.0 to be able to comment.
Many sequencers now have latency compensation(Ardour is included in this) so that the only true time audio latency is a problem anymore is for live performance.
My current audio workstation for examples sake I run at <3mS latency. That is lower than the human ear can really hear, it can tell under certain circumstances that it is there, particularly with two identical audio signals, and delays that small are used for positioning of audio in live performance sometimes(Delaying Front Fills vs Stage Sound vs Center or Stereo Clusters) but for recording purposes not many people run lower, though I have run mine at 1.5 before jsut for grins and giggles and could probably do lower.
The other machine I run it on is my laptop(Mac OS X G4 Powerbook) for some semi-portable recording work I do on occasion(Typically recording voices and the occasional sound effects for a show). Anything I need a truly portable recorder for I have a marantz for, though I am looking to replace that because of noisy pres, there is an interesting linux based portable workstation coming out I may look at, if not Ill probably go for a HD-P2 or something of that quality for my next recorder.
On my workstation I use an RME card to get the audio in, I used to use a Roland interface, but had a few problems with stability at 96kHz so I replaced that. Considering it was one of the first to do 96k I was pretty happy. I tend to record straight to HD, If I can I use a seperate HD than my system drive, ideally I would use two HDs, one to record to and one to playback off of, but at the moment I dont have that set up. Ardour does have a nice capability I havent played with yet to take a JBOD array and turn use it as a RAID array for your recording session, woudl be fun to play with but I havent done it yet, I have heard good reports from some people that have.
I have considered external recorders, but for the work I do, a HD is a much better bet, cheaper, safer, and easier to work with. I would(And do with my Marantz for 2 channels) be using external recorders to do live recordings though if I didnt want to haul my workstation in, and my laptop wasnt availiable. They are nicely set up to do 24 track live recordings for one like the Mackies etc.
For the record obviously Ardour is not the only tool I use. What I use depends on the application, I have ProTools, but I consider it vastly overrated anyways, I do however use it for audio for video projects I do, though the next one I am going to try xjadeo slaved to Jack TC to do that in Ardour and see how that works.
I also use ReZound and Audacity for my single file editing, ReZound is a nice program, unfortuantly only maintained by one person primarily and very slow in development as a result, though he does an excellent job. Obviously I have a high opinion of Audition 1.5 and earlier version for single file editing, and they are great basic sound effect cue builders. The other thing that Audition and Audacity both have, and I believe slowly other software is starting to add, is frequency spectrum based editing for single file editing. This is one of the coolest features and most useful at times for myself personally.
Now the one thing that has to be brought up is running VSTs in Linux. I have a Mac on hand specificly for this I am going to start routing audio in/out and use as a VST server myself. The reason for this is that thanks to steinberg's licensing, it is somewhat difficult(Read illegal) to include native support for VSTs in most open source software. Along with this is the need to osupport the GUIs of the VSTs, which is typically done thorugh software called WINE, which translates native system calls for windows into the appropriate LInux calls. While it works decently in a lot of instances, it still is not a garuntee that it will work, so whatever doesnt work on Linux for me I will be running on my Mac. However taking a look around lately I have seen deecent luck with many that I use or will be using(Such as Kontakt) so I will try the linux approach first myself, but it is tough for many newcomers to linux to understand, and even considered difficult by people that have been around a while.
So in other words, running Ardour and Jack on a Mac can be a good balance at times for those that dont need the flexibility of linux(Or headache depending on who you are