Hi Hammersfan, we bought at off-plan at Playa Diamante, El Mojon through Golf Invest in 2001. The house took the usual extra half year to complete, but we had no problems. The build quality and the materials are very good, they were proactive with queries and generally very helpful.
The only issue was the final payment. At the last minute we were told by the sales rep in the site office that we could only pay a certain percentage of the final payment by bank transfer, with the balance in cash. They would not move from this. When I asked my solicitor to sort it out he said it was normal in Spain and nothing could be done. I called it legal fraud - he agreed. We did complete but very reluctantly with those conditions.
I don't know if this would happen to you but it's worth checking out. Read the threads on this site re problems and tax implications when this happens. Hope that's useful, good luck.
Jane