I have been following the Inkscape forum for several years and the program is becoming stronger and stronger. As I speak it is bringing out version 0.45. It uses SVG and can certainly create (vector) cave maps and is very useable, depending on the level at which you want to use it.
Big and complex files? -yes. It opens the sample map (KAUA-NM w2d mrg.svg) from WALLS and can handle that fairly compex map. I would not advise on starting to learn the program with that file though since it was not prepared in Inkscape.
If you want to do roundtripping you are out of luck with this program at present. So you cannot roundtrip with WALLS using it.
Standalone maps are quite feasible though and you can certainly do your data reduction in WALLS and then do the final draw in Inkscape.
While I am pretty familiar with Inkscape I do not use it for my day to day cave mapping. I use Xara Xtreme for that and find it much more friendly and easier to use. The main reason is that the import/export facilities are much more developed at this time.
PDFs from Xara are easy whereas they are not from Inkscape.
TIFS from Xara are easy whereas Inkscape uses PNGs.
Xara is used by quite a few cavers and there is a fair bit around the net on it. However, it is a Win program so maybe this thread is inappropriate.
Since Inkscape is open source and under active development it is well worth keeping an eye on. I do not see roundtripping on the roadmap but the facilities very much depend on who is interested in working on what. If a caver/mapper was interested in coding that ability into Inkscape then it could be in very soon but that is a rarish combination and I'm not holding my breath.
Otherwise it will do maps equal to the big ones that are around. remember though that the time invested in doing a big vector map is such that it is a bummer if you then want to use different programs. SVGs ain't all SVGs or something. Making and handling SVGs in CorelDraw for instance can give you major hassles if you try and use the SVG file in another SVG program-and vice versa.
The following site
http://sss.boldlygoingnowhere.org/html/Maps.htm shows a couple of maps of simple sandstone caves produced by with Xara. The maps are jpegs, originals are vector.
Erik