Thank you very much for sharing your experience and providing advice. I really have to get a pair and try them out soon! Despite not being fast, I would not refrain from using them in 800/1500 races if they actually helped to shave some second off my final time... maybe you could reconsider your position and wear flats only from 3000/5000 onwards.
I think the jury is still out on barefoot running for Masters runners, another prevailing school of thought is that older runners need all the cushioning they can get. Also you are actually pretty quick for your age. I was a successful collegiate runner and trained at 42 years of age. I ran 4:40 but I would say the majority were over 5 minutes so you are good at around 5 minutes and fast enough to occasionaly were spikes. I would recommend 90% of your running in training flats, not racing flats and try out the spikes gradually. I think the spikes would be useful in the half mile, I think negligible advantage in longer distances, but you can make that call.