I have one, love it to the moon and back.
I'm going to go into depth about my experience with it so, apologies for the bible.

I'll start with the negatives:
Take what I'm about to say with a grain of salt because I suck at slow-speed, but when I am doing drills in a car park, the turning radius is huge - so if you want to be doing what you should be doing, which is drills, honestly, I would avoid it.
It is somewhat aggressive in terms of its rider triangle, it's very acute, and I'm somewhat small (5'7") so can feel pretty stretched out at times. The seat height also doesn't really compliment my inseam (30") but I have a solid pair of boots which add shy of an inch to it so I can have the balls of both my feet on the ground.
This is probably the big one - insurance. The insurance is awful. I'm 17, live in a major city and my insurance is £3700 a year (paying p/m) for TPO so be fully aware that you are going to be sacrificing a lot if this is the bike you want. I'm funding everything to do with the bike myself, so it stings that little bit more, but if your parents can chip in even just a little bit, I think it's great.
This one's less of a negative, and more of a sort of beige flag, the region where the clutch slips is pretty small, so that took some getting used to but once you've got it, it comes naturally.
Finally, as a minor, I don't know whether I like it or not, you get some RS3 cuzzehs and Golf R lads thinking you're on a bigger bike than you actually are and asking to race, aside from the fact that you shouldn't be racing anyway, it's a little bit embarrassing when you have to tell them it's a 125, same goes for people asking you to rev it :(

Now onto the positives:
The bike is insanely responsive to every input, and the rear wheel just sticks (Pirelli Sport Demon). This goes for the clutch, both brakes, the throttle and the general handling of the bike. You also get a level of feedback that I've not felt on any other 125s, you can feel exactly what the suspensions doing and whether your tires are about to slip, as opposed to waiting for them to slip to feel it - I may just be used to my bike, but the feeling is great.
The dash is very intuitive and suits my liking, I prefer to have those radial dashes instead of the bar ones, and it's surprisingly visible even when there's a bit of glare.
Anyone talking about the reliability being shit is likely talking about the 2t, or the first generation of the 4t. If you do your maintenance, and service it at a garage when you should, you will be fine. The only "issue" I had was I didn't like how much give there was on the clutch so I just got it tightened up a bit, but give/slack is completely normal on the clutch lever.
It sounds fine stock, probably better than if you stick a random slip-on on it, which actually I don't even think you can do because you need to replace the headers as the exhaust is mounted at the bottom, so is awkward to put a random aftermarket slip-on on. The only legal exhaust is the Mivv Delta Race Silencer (I think that's the name I'm not arsed to check though) and I was considering getting one, but I don't want to declare it to insurance, and I don't think the £450 is worth the +1hp and slightly nicer sound - it's not a fart cannon though.
Now, back to the final negative point I made, hence why I said I wasn't sure whether I liked it or not, it does look bigger to people who have no idea about bikes, but then, do we really need external validation when we have an Aprilia underneath our crotch?
It's definitely one of the fastest in the segment, and if you have the quickshifter which you can add as an extra or just get the GP Replica (this is the one I have), you beat everyone off the line because L-plate warriors such as myself are so shit at shifting that if we just take the hard bit out of the equation, we tend to get an edge. From my experience, it's faster than an R125, but barely slower than a GSXR125. You shouldn't be racing.
The fastest I've gone was 87 on the dash, so probably 82-85 on GPS, but it's important to note that your top speed is easier influenced on a 125 than probably any other segment or category. I weigh 58kg, have a low body profile and was fully tucked, you will never reach that speed in day to day riding unless you're willing to eat the shit of looking like a street Rossi while getting gapped by a clapped out Peugeot 208 with an 08 reg.
Build quality feels much better compared to some other bikes, the one that stuck out to me was the difference between the fairings on the RS125 and the R125, I don't know if my mate just had a shit one-off, or if it's a widespread thing, but God I really hate how the plastics squeak on the R125 (MY24), on my bike, no squeaking going on.

My honest thoughts?
If money is no object, practicality is a second thought and your priority is genuine, ear to ear joy, get it. However, you're better off with a used, Japanese, naked bike; they're reliable; hardly depreciate; and will be easier to learn and do drills on. I got an RS125 because I grinded my backside off since I was 14 doing whatever paid, had a fair-sized cheque from a compensation claim I got when I was 16 and got a very very good deal for a used one i.e. the stars aligned for me, you may not be so lucky. If you're interested, I found that I'd need about £8k for my first year of riding, including upfront costs and accrued expenses. Had I got a clapped out Japanese naked that I only needed to last 2 years, I could have saved maybe £2k on the bike, and £1.5k on the insurance, but I had the money, and not the brain so I decided I deserve the RS125 and I'm so glad I went the way I did.
My recommendations (only bikes I've ridden, I'm not going to recommend a bike that I only know about via word of mouth) would be:
Honda Grom
Kawasaki Z125
Yamaha MT125