The funny thing is when searching this question on the internet the first google results page is about sound device company, I am really interested in the daily experience of other sound recordists.
I have rented many time the 664 and now I am planning to make a purchase, mostly I do documentaries and tv show, sometimes feature film, the 633 for my needs could be fine, but having 6 xlr inputs is indispensable.
Remains the 664 and 688, those 1200€ more are really worth it?

Depends how often you need the extra tracks. I used a 744/302 combo for many years and rented a 688 or Zax Deva when more ISOs were needed. The 688s preamps and limiters sound 'different' then the others, so going from one to the other is unsettling. Don't know about the 664. I liked the Dugan auto mixer on the 688, but the 664 and 633 have it now as well.

have you ever used a 633 on a feature film? 2 boom and 4 radios. as maximum setup.

Depends how often you need the extra tracks. I used a 744/302 combo for many years and rented a 688 or Zax Deva when more ISOs were needed. The 688s preamps and limiters sound 'different' then the others, so going from one to the other is unsettling. Don't know about the 664. I liked the Dugan auto mixer on the 688, but the 664 and 633 have it now as well.

the 664 does not have the duncan.

https://www.sounddevices.com/products/mixers-with-integrated-recorders/664/feature-comparison

I thought it was available with a firmware update? .. like with the 633.

I thought it was available with a firmware update? .. like with the 633.

No. Remember the 664 is the oldest of the 600 series mixers, and has a more analogue design than its newer brothers. So I predict the 664 will never be capable of Dugan automix. (& not the only limitation either)

Thus I recommend instead going with the newer 688, or the 633 if you can live with its lower track count. (on the flip side... if you're looking at 2ndhand prices, you could get a 664 for as cheap as a 633!)

I am considering that the real benefit to going on the 688 is the SL6 superslot

I found and purchased a second hand 664.

May I ask, what did you pay for a 2ndhand 664?