I have one that's spalted mango. It's great. I think Pepe Romero makes good stuff, innovating rather than following trends.
Some thoughts about my replica:
I haven't tried to measure, but it seems to have the longest sustain of my ukes. It has a more guitar-like feel, although, I'm not sure where that's coming from. Maybe the wide fretboard. On mine, the action was set up a bit too low. No buzzing or intonation problems, just not as comfortable for finger picking. It still plays great for fingerstyle, which is my preferred kind of playing, but it strums really well too, which in my experience ukes seem to sound better for one or the other. This one does both pretty well. It has 12 frets to the body, which I like as a contrast to the popularity of longer necks. You give up a little playability up higher, but I think it enhances the tone greatly.
Speaking of tone, the several different replica models I've played sound more similar, one to another, than just about any other uke. Having played or heard the different versions of the replica, I'd say maybe the spruce/walnut one stood out as most different from the rest, and part of me wishes I'd picked that one up instead of the mango. Still, the sounds are very similar among the different tonewoods, and the mango replicas are probably the least mango sounding of all the mango ukes I've heard and owned. I think most people would be really hard-pressed to distinguish (much less identify) different tonewoods in a blind test with replicas. But aesthetically, I liked the spalting on mine, which can be hit or miss.
Aesthetics-wise, I like the all black UPTs on the newer models (mine's silver), and dislike the abalone perfling. The spanish heel is unique and looks nice. Instead of a typical heel cap, the back of the instrument is built to cover the heel. Looks pretty neat. Despite being impressed with a lot of Pepe's design choices, I can't stand his regular headstock, and greatly prefer the one on these replicas. I think it's unfortunate that just about none of his other instruments use this headstock.
Something I became aware of recently. I like the feel of matte or satin finishes, but tend to prefer glossy finishes for looks. Unfortunately, I often have problems with the stickiness that occurs from skin contact with glossy finishes. I have this issue on a number of my instruments, and just realized I've never had that issue with my replica, which is glossy. I don't know if this is something that varies greatly from person to person, or if the finish Romero uses is less reactive with most people. But I just noticed I never have an issue with the Replica, and I'm really happy about the finish. FYI, most replicas are glossy, except their (discontinued?) spruce/acacia model which was apparently meant to be a laminate version, but a mixup resulted in a production of solid woods.
Some last thoughts. Since you asked about spruce vs. mahogany: although I have a couple all mahogany ukes (a Martin and a Larrivee), I find that I tend to prefer spruce tops. But like I said, these replicas sound very similar despite their woods.
I think because of the design choices with the neck (wide, flat, and short to the body), this is probably one of the most different ukes in my collection. Especially playability wise. After playing the replica, I feel like I'm returning to regular ukes when I go back to my other ones.
Hope all that's helpful.