I think quite a few respondents are losing sight of the DIY aspect of all this. For DIY read - "some people only do this once every year or so - or perhaps only once".
I appreciate that copper is much cheaper, and that the basic 'end feed' joints can be picked up by the bag full at plumbers merchants for very little money. They are simple in construction, neat in appearance, and virtually all professional plumbers use them. For a professional, the cost saving over a job is considerable and a good soldered joint should last 100 years. Once mastered (!) the technique becomes second nature.
However for the DIY person ease of use can justify a premium cost, especially if only a few units are used. As discussed in this thread, successful soldering depends on care, careful preparation, and a learned technique to get the heating of the joint just right. In certain circumstances (such as soldering under the floorboards next to joists) there is the additional risk of fire, so you have to take extra precautions. Finally you are dealing with (ouch!) heat above water boiling point. It also involves quite a bit of extra work in the situation when you realise that you haven't put the pipe work quite where it should go (or realised you should have put something else in first). Or if it turns out that your joint wasn't as good as you thought it was. If you have already filled the pipes with water your problems can be compounded by having to dry everything out first. I value the ability to quickly un-pipe runs and then re-fit them.
So I think we are getting a little at cross purposes. End feed copper is cheap and proven by time, and I would suspect will still be around in another 50 years. The professionals choice. As mentioned elsewhere, when compression joints first came in the traditionalists predicted dire problems but now they are accepted. I expect that there will be more and more use of push fit joints, especially around units such as power shower pumps, where ease and speed of fitting justifies the higher component cost. Perhaps one day your average plumber will appear with a length of plastic pipe and a pair of snips and the smell of hot flux and solder will be gone forever. [O.K. I know boilers need copper close to them :-) Grant me a little poetic license.]
If cost is the overriding factor then take the time to learn to use soldered copper. It is a useful technique anyway for visible joints you wish to be unobtrusive. For dedicated DIYers the technique can be as important as the end result. However if you are doing your first (and perhaps only) plumbing job then push fit joints could save you a lot of grief.
What prompted the original post? I had just put together a run of four pipes which all go round one corner of the bathroom. There is a little 'nexus' with six elbow joints in a very small area. With Speedfit I just fitted whichever suited and then threaded additional pipes under/round/over the first one. Just a push, and a leak free joint. I thought 'damn, that was so easy'. And reflected on my previous forays into plumbing over the last 30 years. Which led me to the conclusion "I LOVE Speedfit!"
This is an expression of a personal view, does not denigrate the use of other technologies, and freely acknowledges the right of all thinking beings to use the plumbing techniques of their choice :-))
Thanks again for all the responses.
Cheers Dave R