What herrman says... Yes I did ride an Aprilia RS125 for 3 years before going up to the S - Tony did the same.
We had great fun on the Aprilias, derestricted, because of their light weight, they are very quick (over 100mph if you want it). They were an excellent training bike for what we have now. I think you should honestly get something at least mid range before you get on a Boxer, not because of the difference in speed/acceleration, but mainly because of the difference in weight.
But there are down sides: The changes in DAS are well worth looking into for a start.
They need to be warmed up for 10 minutes before revving them, and due to the way you will ride them, that means in practise standing there with them stationery for that 10 minutes - including in Winter.
They do need expensive 2 stroke oil and that just about doubles the cost of your fuel.
You also need to be looking at new rather than 2nd hand; I'd say that our 2 bikes were probably the only 2 in the world that had been cared for properly! I wouldn't risk buying 2nd hand.
They will teach you a lot about throttle/clutch control.
On the plus side, they are 5 times the fun of any other 125cc that you could mention - I cant overstate that point.
They will let you have a good idea of how a big bike is going to handle/U turn/brake etc.
They will teach you loads about proper gear changing.
You will beat some of the bigger bikes on the twisties - i promise!
They are reliable if looked after properly, despite what anyone would tell you. Most of these bikes are ruined by young tear aways, but serviced and warmed up properly, ours both did 10K without a spot of trouble from either of them.
They can be difficult to set off on - that sounds weird I know, but it is easy to stall them, being 2 stroke (but see the plus side re: clutch control!!).
I still think that a good option would be to get through DAS then look for a ZXR400 or an SV650 maybe.
Like everyone says. The choice is yours and you can only take in any advise you find useful then follow both your head and your heart!
Pauline
PS Anyone who tells you they didn't follow their heart when buying a bike is talking thru' their posterier IMHO. It is very much an emotion and passion thing, otherwise we would all be in boring old cars.
PPS Yes you are right - the Aprilia RS IS a sexy bike!
Pauline