I have a 1Tb fusion drive.
And for my daily use, it's near perfect.
Of course there are faster solutions, but my iMac is really fast enough to forget I have an HDD inside.
The only limit is when using Parallels Desktop: it's completely on the HDD portion and you feel it's really slower (I don't use it very often).
But for desktop applications, internet, music, Xcode, image processing, it's perfect.
Fusion drive is really well optimized, and it doesn't keep all the OS and application files, it only keeps on the SSD the most used file blocks. And with 24Gb you can cache a very important part of your activity.