The best endorsement you can give a product is highlighting the fact that some of the world's best players choose to use it without taking a penny (or cent) from the company who manufacturers it. Even better if they then go on to win back-to-back US PGA Championships and US Opens. And that's what has happened with Mizuno over the last three years.
Mizuno has always had a reputation for producing some of the best players irons with their MP range, but it is their 'game improver' offering where they have made monumental strides. The term game improver is perhaps a little unfair when describing the current JPX range as (at the time of writing) the world number 1 is using them. These irons feature more technology than the straight blades in the MP range, but for reasons stated above the lines are getting increasingly blurry between at who they're aimed.
The JPX range or family replaced the MX ranges of irons a few years ago and are Mizuno's take on a game improvement irons - designed for players who want to play the best but can't play the irons in the MP range. As JPX irons have evolved the technology required to deliver forgiveness has streamlined into a package which competes with the MP line on cosmetic appeal as well as performance.
When looking at second-hand Mizuno irons, you'll notice there are a few different options - Hot (or Hot Metal), Tour, and Forged. The hot metal irons are the biggest in the family and are packed with technology and materials to deliver unrivalled distance in a premium package. The forged models are constructed in the same way to the premium MP irons but still feature the same forgiveness technology associated with the JPX range. Finally, the Tour model is the smallest and one which blurs the lines between the JPX family and the Mizuno Performance line the most - and the model played on Tour by a lot of Mizuno staffers, and non-contracted players.
As the JPX lines close the gap between themselves and the MP line, the MP line is beginning to come the other way and feature more technology than ever before. The MP-20 range (still features a pure blade) but the other two models in the range are both packed with 'traditional' game improvement technology, just bundled up in a stunning Mizuno package. The MP-20 MMC has tungsten weight in the sole to get the ball in the air quicker while the MP-20 HMB (Hot Metal Blade) is a hollow body iron which delivers extreme ball - something not seen before in an MP iron.
Irons in the MP family are hard to keep track of as the number jump around a little. Thankfully, due to the nature of their design and construction, they have not really changed much before the MP-18 and MP-20 (released in 20118, and 2020 respectively).