My own experiences:
In September 2010 I drove a car - Citroen Xantia 2.0i - over to Spain with the view to keeping it there. It had 12 months MoT, 12 months road tax and12 months fully comprehensive insurance. I checked with the insurers about the legality of keeping it in Spain for the year and they were quite happy provided that I could show that neither myself or the named driver used it for not more than 60 days in the 12 months by keeping all return flight details. I deliberately chose this car because a) I like them, and b) there should be plenty of them around continental scrap yards for parts etc.
Over the following year he became known affectionately amongst our Spanish neighbours as 'El rana Inglese, 'cos he is green, French and on British plates! Then, in August this year I took the plunge after taking some very useful advice and info from Faro on this site - thanks for that, Faro.
Firstly, I went to visit a local insurance agency who gave me Spanish fully comprehensive insurance on the UK number - all I needed to do was go back after re-registration and inform them of the new number. I was very pleased to note that he is now covered FC, 24/7, Europe-wide, for ANY driver over 25, with all-singing, all-dancing Europe-wide roadside repair/recovery, and for LESS THAN 50% of what I was paying in England for insurance only for two drivers!
Next, I went to the local autosdesguaces and bought a pair of LHD headlamps for the grand total of 50 euros. Just three screws each side and these were fitted in a couple of minutes. Double rear fog lamps are standard on this car so nothing needed there. I then found a perito for the engineers report - he didn't even need to see the car, just the UK V5 document.
Then off to the Itv station - my first little slip-up! Although the Itv is nowhere near as stringent as a UK MoT, they are very keen on tyres. Both tyres on any axle must not only be the same size and construction, they MUST also be the same MAKE! Although both my rear tyres were perfectly legal, there was one Avon and one Firestone, so off to the tyre depot for a new pair. We then sailed through the test with no further worries. As this was a test for re-registration, the docs and paperwork had to be checked and signed by a senior engineer so I had to go back the next day to collect them Remember to put the red sticker in top o/s corner of the windscreen..
Now down to the SUMA office to pay the road tax - as usual, remember to take originals (and plenty of copies) of passport, NIE, padron or residencia etc. Into the bank to pay the cash over and then to the local revenue office to sort out the import duty. Queue up, get the paperwork and away to the bank again to pay the cash and get the all-important blue stamp. Back to the duty office and exchange for more docs with more blue stamps.
Over to Traffico, pay the registration fee and queue again to hand everything in. Get a reference number (with blue stamp) and told to return next day to get the number and Permisso de Circulation (I think that's what it's called) - the Spanish log book.
Back again to Traffico next day, hand over my number, collect all the relevent papers and, at last, across the road to the number plate shop for the new plates. I wasso excited I even took my cordless drill into town with me and fitted them there and then!
In summary: dead easy to do so long as you have the time, patience and can put up with the burocracy with a smile. Yes, I would - and will - do it again. Excluding the insurance and tyres, the total cost was less than 500 euros, a fair chunk of which was the import duty - I now know to do the job earlier next time and avoid much of this. Also, as the tax and duty calculations are based on the manufacturer's stated CO2 emmissions figure, the next one will be either a diesel or a smaller, more efficient petrol engine which will reduce costs considerably, but these are all things you learn from trial and error. I also find that Spanish drivers seem to treat us with a bit more respect - less hooting and carving-up at junctions!
Incidentally, on my way home from my second visit toTraffico, for the first time in five years I was stopped in a roadside check by the Guardia Civil; just glanced at the red sticker and waved me on!
Hope this may be of help to those of you who want to try.
Steve.