Hi Chris, I started off turning with only carbide insert type tools but now I also use Hss a lot. It's kinda like horses for courses, if you know what I mean. I have a round profile insert tool which gives an amazing almost-mirror finish to brass and Ali, but obviously cannot turn to a sharp edge. In fact all my insert-type carbide tools will not give me a good sharp edge into a shoulder.
Another difference I have found with Hss vs carbide tools is that Hss will allow you to shave off a whisper of a cut, like 0.001-0.002mm if you needed, but on my C2 ( which, lets face it, is made out of a 50-50 mix of industrial grade pasta and used post office elastic bands, so this may not be true of better, or better tuned, machines ) using indexable tooling is a little less precise, ie you can wind on a cut of 0.02mm and it doesnt cut to that depth, as far as I understand it, the carbide does not have as sharp an edge on it and cuts by brute force rather than by sharpness.
That's fine for roughing out, but when you take finer cuts, I've found on my lathe when winding on 0.02mm per pass, the carbide will take virtually nothing on the 1st pass (probably partly due to the flexing of the rubber machine) , then will take more than the 0.04 ( 0.02 1st pass + 0.02 2nd pass ) on the 2nd pass. This makes fine finishing cuts very unpredictable for me, so I tend to use Hss for the final finishing cuts.
For roughing out and heavy cutting though, indexable tooling is great, it'll hog out a deeper cut and leave a better finish while doing it (in my very limited experience) I've found on some mystery steel I have that indexable tooling gives a better finish on when taking a heavier cut (0.12-0.14mm) than lighter cuts (0.06mm)
So I guess what I'm trying to say is that indexable tooling does have a useful place in my workshop, if looked after it can last well, but the tips can be expensive. But they wont replace Hss tooling, both have their uses and places.
I'd reccommend getting the 6mm round profile tool, it has been the most used and most useful out of the indexable tools I have, and I am only on my 2nd tip. ( it's great, when it gets a bit blunt or damaged, just rotate the tip around a bit to expose a fresh cutting edge

)
Hope this rambling tome is of some help.
Tim
EDIT:
I forgot to mention that if you dont have a QCTP, you can get some indexable tooling that has the cutting tip the same height on each tool, handy for reducing setup time. My 6mm round profile tool and my standard LH turning tools can be swapped without adjusting the height setting at all. I got them both from RDG, they are both 6mm shank.