Due to a project I was working on, I had need of the Korg M1 sounds--and the Legacy Collection had the whole M1 offering (great!). This eliminates the need for additional hardware ROMplers, and ROMplers generally don't lose their integrity when converted to software like analog synthesizers do*. And since the M1 was part of the Legacy Collection I also got a host of other plugins as a bonus; after working with the plugins for awhile it became clear that they've had striking features that MOST softsynth producers ignore. AND to top it all off, these plugins are of such high quality in every aspect. Oddly enough I have never had exposure to Korg synthesizers prior, but I've always held the Mono/Poly in high regard as it's capable of many powerful sounds.
*I am aware hardware can affect the resulting filtered sound, such as the differences between the JD-990 and JV-1080
First lets go through some of the advanced tuning capabilities that all of the Korg Legacy Collection plugins have.
Master Tune goes from 420 Hz to 460 Hz (of course default is 440 Hz as expected). Custom temperaments can be programmed in! The predefined options are not that great, but it wouldn't be too hard to key in something like meantone. This is the first time I have seen custom temperaments aside from Modartt's Pianoteq. I suppose EWQL occassionally has tuning options available, but they're not as flexible and not applied to all of the plugins.
I should also add that these are probably one of the few, or only plugins in the world that allows legato and portamento with a custom temperament.
While some of the plugins are required to be preset-centric (such as the M1 and Wavestation since they're ROMplers), even the Mono/Poly and Polysix have well-programmed presets.
One of the first things (like many plugins) that comes to mind is that the interface is fairly small; fortunately it's still quite easy to read as Korg ensured the fonts were *clear* and in many cases omitted anti-aliasing (which in the case of many soft synths, actually causes aliasing by removing clarity from the tiny fonts and instead blurring them over). So despite the small interface I'm still always able to read everything.
Unlike the M1 VST, the Wavestation softsynth doesn't have handy "category" browsing. So if for instance I wanted to take a look at "strings" based presets, I'm not able to do so. In thsi regard, it's probably a good idea to go through all of the Wavestation presets and make note which ones stand out to you that may be useful, and create your own categorization.
Editing effects on the M1 VST is a little arcane, and sometimes I find when modifying reverb or chorus, it doesn't immediately change anything. Of course with some more practice I'm certain the interface will become more usable, but it is a little messy and could have been better organized. There's also a weird glitch where sometimes going to the 'global' button, you cannot immediately go back to 'browser' and instead have to click something like 'prog' and then 'browser'.
Korg absolutely put amazing effort into these plugins, and it shows they paid attention to many details. It's also a great way to preserve the history of their old ROMplers. I think anyone who wants to expand their sound library or is starting out, should definitely get the Korg Legacy Collection: at the very least you get a suite of high quality plugins for a very reasonable price. Despite the very few minor issues, it doesn't affect the overall product.