I have two vintage Martin sopranos, a Style 0 from the 60s and a Style 2 from the 20s. I've had a fortune to be able to play a Style 3 from the 30s pretty extensively and a Style 0 from the 20s for awhile.
Over the last few weeks, I've played all the recent and new Martins, from the S1 to the new Nazereth-made style 2 and 2K sopranos, concerts and tenors. I've also played the Mexican-made Style T1K Tenor.
Here's my critique.
The Mexican made S1 was poor; not up to Martin standards. It was thick and overbuilt, and though it was cheap compared to the new Martins, it just wasn't as good as an Ohana-made uke that cost half the price. Get an SK-35 or even a CK-70 to get a better sounding uke at 2/3rds to half the price.
The new Nazereth-made S2 ukes are very, very good. I think they are comparable to Kamakas in sound and feel. They're also comparably priced. In short - they're great ukes, especially the koa ukes. They're lightly built, resonate well, have great intonation all the way up the neck and they feel great. I recommend them, if you have that kind of $.
The Mexican-made style T1K is less than $500. It's worth every penny. It's the best uke in the $400-$600 range I've played. If I ever feel the need for a tenor, I wouldn't hesitate to pick one up.
Do they compare with vintage Martins? No. Vintage Martins have a construction style and lightness that even the best of the new ones don't have. No one makes them like Martin used to make them. Also, no wood is going to 'open up' enough to approach the same sound. They're different animals and if you want the sound of an old one, go buy an old one. You can get a style 0 or a style 1 for $600-800.
But, that said, new Martins are really something great now. They got the formula right and are producing excellent instruments, and despite all the chatter about how "they're failures they're too expensive, they aren't worth it," I will wager a lot of that talk is from people who "heard" they weren't as good as the old ones and who probably haven't actually played one yet, or only played an S1 from Mexico. You really have to try a new one and judge for yourself. A friend tried out a couple - he had a different opinion than I did. He felt they were great, but too costly. He isn't wrong, but I felt if you had the money for a Kamaka, these were a viable option.
I especially recommend the Mexican-made T1. It's a really really good uke and excellent value for the $.