My first uke was a cheap Mahalo I bought on a whim after seeing it in a shop window. I knew absolutely nothing about ukes when I got it, I just picked it without even trying it because it was a cute purple colour. Needless to say it wasn't that great but on hindsight it was actually quite ok for a low-end uke, and it opened my appetite for learning how to play it, although I didn't really touch it for about a year.
Fast-forward a bit, I finally start learning how to play and realise the action is too high and the strings hurt my fingers. I get the action lowered at a uke shop and change the strings to Aquilas based on people's advices. Massive improvement but not quite there yet. I could tell it wasn't that great of an instrument compared to others, it had buzz, some minor intonation issues, and the C-string was booming too loudly. By then I had decided the ukulele was the instrument for me, so I did a lot of research in order to find a better one for sessions, while wanting to keep the Mahalo one as a beater for my couch.
I looked online into all the mid-range ukes from Kala, Ohana, Lanikai etc, for budget reasons. I tried lots of models in local music shops too, trying to look for a good sound and playability. I even bought one and returned it an hour later because it still wasn't quite right: it had a weird resonance frequency I didn't like, and like most of the ukes in that price range (<100pounds) it sounded a bit too boxy for my taste. I realised I needed to research some more before buying. So I did, for weeks and weeks and weeks.
Finally, I went back to the little shop out of town that had lowered the action on my Mahalo, had a look at what they had and tried a Kala KA-SS (the solid spruce top cousin of the popular KA-S). Love at first sight. It had a great open and bright tone ("like the voice of a thousand angels" I said to myself), perfect intonation, and the action was as low as you cam get without a single buzz, thanks to the shop owner's excellent set-up. The price tag was very good as well. Bingo!
I kept the Mahalo as a beater for a bit then gave it away to a friend but I still use this Kala as my main pub session uke. It's a cracking little lass. Never getting rid of it!
So in short: 2nd uke = upgrade from cheap laminate to mid-range with solid top, good intonation and proper set-up. The first one (and lots of research) helped me figure out what I liked and didn't like about ukuleles, what I needed and didn't need. 2nd was a perfect fit but it opened the door to UAS, because after that I needed a banjo uke too, and a low-G one and a....