Regarding joypad Boomerang B103 we're talking about here, there's review on youtube:
Bitte melde dich an, um dieses Medienelement zu sehen.
You can see there how precise is this joypad with joystick add-on testing on Last Ninja game.
Regarding this review, I would like to point out two things:
1. Reviewer complaints about short cable. Reason for this is that he bought economic edition of Boomerang B103E (cable length is 1.2 meters), and luxury edition B103L has really long cable - 2 meters to be precise.
Spanish link I gave you in this thread for buying this joypad is for luxury edition (that includes two add-ons - spherical dome and joystick add-ons for d-pad, for economic version you need to buy these add-ons separately).
2. Reviewer forgot to mention that turbofire button is lockable, so you don't need to hold your finger on it constantly in shoot'em ups 
All other solutions for using new joypads on C64/Amiga are double priced, because you need to buy bulky adapter and joypad for other consoles.
Just like I said, PSX adapter with PS2 controller is best solution.
Best thing is - you can choose to use digital or analogue commands on PS2 controller - they work at the same time
CapFuture1975 can confirm that.
But this solution with Boomerang B103L one-button joypad will satisfy almost all C64 users wanting joypad for playing games, knowing that C64 doesn't support second fire button.
It's ergonomic, economic, very good and doesn't need adapter to work.
It's not perfect, but for sure deserves 4 out of 5 stars.
If you want to use it on Amiga with games supporting two-fire buttons, forget it - second fire-button is not supported on this joypad.