I have just come back from a holiday in Colombia and noticed the conversation about Merlin on the blog. We tried Merlin at the Hacienda we stayed in, which had an extensive bird list, and at first it did well. All the birds it found I could also see or hear and most of them were not actually singing, just doing contact calls. But then it seemed to go off on flights of fancy - it found Chestnut-headed Orependola, and both Yellow-legged Thrush and Pale-eyed Thrush, none of which are known at the property whereas it missed Black-billed Thrush which is common there.
And we tried it in our garden on 6th May last year - it found Greenfinch and Goldcrest which were definitely not there. (I can still hear Goldcrest calls - there weren't any- and Greenfinch have been regretably absent for about 10 years)
So the verdict - surprisingly good but it makes big bloopers as well. I think it just tries to hard too identify every little squeak. (I wonder if the Orepondola at the Hacienda was actually noises from the kitchen!)
On thing which is very useful is that it highlights the bird that it is hearing at any point in time so maybe ir could be useful for learning bird sounds and calls.
Rosemary Royle