Pianoteq fulfills two unique niches: a small footprint for a reasonable piano emulation, and providing options for alternative tunings. Despite the world operating at 440 Hz and equal temperament, there can still be a need for different tuning systems if you're exploring historical music (particularly baroque).
The stock piano packs may be leaving more to be desired, so I would recommend getting the Bluthner pack at the very minimum.
The version I recommend (and use myself) is Standard. If you're looking into custom temperament functionality, don't get 'Stage' since these parameters are omitted. Professional is probably overkill unless you're after 192 Khz and overtones: both of which I think are not necessary.
I do enjoy the EQWL Pianos in regards to the realism, especially the Bosendorfer since it's extremely well done in my opinion. However, EWQL pianos *does not* give you custom temperaments, and any custom pitch will have to be modified after the sequence is rendered. Furthermore EWQL Pianos is *huge*, although I think the argument of storage is largely obsolete-- unless you're running your music production environemnt on a laptop that has soldered-in storage. Which... nobody I know does such a thing. A better argument to make is that EWQL Pianos will take longer to load than Pianoteq due to the sampes that must be loaded into RAM.
Therefore, Pianoteq is invaluable if you want a decent sounding Piano or Harpsichord and configure it with unique parameters. With a sample-based instrument this is just not possible, unless you start sampling every single temperament (which nobody is going to do, because you'd also have to tune and re-tune the piano for each recording session).
I can't really think of a SINGLE thing that's wrong with this plugin. There's nothing wrong with the interface, it does exactly what you need it to, all of the features you need are there, it's expandable to produce any type of sound, etc. The only disadvantage really is the price: but if you think about it I'd say it's justified. The R&D it took to develop was fairly involved, there aren't many recurring costs (unless you're upgrading to a newer version of Pianoteq, and there has only been one version change since I've owned it for many years), and the developers have to pay their bills somehow.
Pianoteq fulfills a unique purpose and does so very well. The software is well made and it (once configured accordingly) is probably the best piano 'emulation' I've heard. Not to mention you can customize the sound of each instrument. It may be an expensive purchase if you're just looking for a 'standard piano', but the *point* of Pianoteq is to be a lot more than a typical restricted piano.