Just as on Windows, the range of plugin quality varies widely.
The first thing to be aware of is that at present, the Linux native plugins hosted by Ardour have no GUI of their own, so visual chrome is absolutely not part of the experience there (there are people who actually prefer this, although I know that many users respond well to shiny hardware-emulating GUIs).
There is probably no equivalent to anything at the level of the Waves plugins, and because of Waves Inc.'s use of iLok for copy protection, there isn’t a good way to run their plugins even with Ardour’s VST support (its not impossible, but its probably beyond a normal user to configure this).
That said, there are compressors and limiters that do a very good job. You might or might not like their qualities - thats always an issue and is often why there are so many plugins in the first place. There are a couple of good reverbs. EQs are harder - I still haven’t found one that sounds as good as the Paris EQ to me, but there are a couple that come close.
The bigger problem is finding the good stuff. The problem is that the plugin sets that are available (for example, swh, CAPS, TAP) contain a mixture of:
- utility plugins where quality is not really an issue
(e.g. splitters, delays, etc)
- high quality plugins
- experimental, unfinished or lower quality plugins
Plugin authors don’t want to remove (3) for a variety of reasons. As a result, when you see the list of LADSPA plugins, its very long and there is no way to know a priori what is good and what isn’t.
On the other hand, this is hardly any different than the situation with VST plugins, with the one exception being that there are vendors who in general never seem to release anything except (2). Searching the KVR Audio website for good quality plugins is much more hard work since you can’t actually just try things out as part of the search.
My recommendation is that you try them out. If there are no LADSPA plugins that meet your requirements, consider using Ardour’s VST support (which is problematic to build because of Steinberg’s licensing). This opens the door to a number of free, high quality VST plugins (I very much like Paris EQ, Ambience and Transverb for example).
The most important thing is that it is absolutely possible to mix and master professional quality music on Linux with Ardour and other tools. There maybe some specific things you cannot do (you will not find POW-R dithering anywhere, for example, and timestretching technology on Linux is either not very good quality or is very very good and very very slow). But just as the hoary old cliches of audio engineering tell us, given the technology used to record Sergeant Peppers or Dark Side of The Moon, the idea that you can’t do production at that level with the tools available already is just silly.