I think it really depends on what you want to do with your mixing in the future. If your MIDI controller is keeping you happy, stick with it - Don't ditch your gear just because of what other people say. A lot of DJs have preconceived notions that you have to pass through some sort of turntable right of passage, as if you can't mix unless you've played with them, but it's all just nonsense.
I don't think you have anything to gain by using CDJs or a turntable over Mixxx, unless you want to dig into scratching. When I started DJing, I bought turntables because I really just wanted to learn how to scratch really well, and I didn't even really care for beatmixing for a long time. Now though, all I do is beatmixing (using Mixxx), and I don't really bother scratching. In fact, my turntables have been sitting in a closet on the other side of the country for the last year and a half. I can't say I miss their bulkiness, as MIDI controllers + my laptop are way more portable.
Some professional DJs prefer lugging CDJs around because they're dependable. You can use Mixxx's vinyl control (or any other DVS) with CDJs to take advantage of your MP3 collection, but if your laptop breaks in the middle of your set, you can still just use the CDJs with regular CDs (or MP3 CDs). That added level of safety is what makes those appealing to people who DJ often. That said, I still know tons of people who DJ with just their laptop and a MIDI controller (or turntables), and would use their iPod as a backup in case their laptop broke.
Mixing is just mixing. It doesn't matter whether you're using turntables, CDJs, or software like Mixxx. Mixxx is cool because you get free upgrades, whereas your CDJs and turntables are stuck with the hardware features that they have. That fact, plus the customizability of MIDI controllers is what really makes Mixxx appealing to me.
Hope this helps! Just remember that even Paul van Dyk was a beginner DJ at one time...