It was the low-end member of the Amiga range for a while, so I suppose it was "the worst Amiga model that you can buy" new at that time. The same could have been said of the A500, A500+, A1200, etc. at other times.
It wasn't that much better (or at least more powerful) than A500, but way less compatible. Many of the old games failed to run on it and I guess that's why it's been said as the worst model. It's quite clear from gamer's perspective, especially before there was anything like WHDLoad and decent upgrades to A600. Many games also had keyboard shortcuts to numpad, which was missing on A600. So, in any case, you lose compatibility with old games and won't get any new games either (like you get AGA games on A1200).
And for productivity use it's was slightly better than A500 (IDE, PCMCIA, newer OS), but then again, it didn't have better CPU or enough memory to be really that much better. And at that time there wasn't drivers for PCMCIA network cards etc, so PCMCIA was quite useless at first.
A1200 on the other hand was much better on all those areas, with new AGA chipset and all.
These are the points why I also consider A600 as one of the most useless Amigas around, if I'd have to pick one. No matter how cute it is :/ A1000 would compete for that title too. I think even A1000 is more compatible with the games than A600, but worse in productivity use.. so it depends which is given more weigth.
But of course you can get it more useful with accelerators and PCMCIA stuff nowadays, but then again, A1200 will still be better for features and compatibility.