I bought the whole set the first time (minus the thicker spacers and no adapter for the 700XL). I love the offset base itself and almost always use it with the DF500. It allows me to ensure that the machine doesn't shift during plunges; in other words, greater leverage and stability during plunging. This isn't a general problem but I have had the DF500 move just slightly on occasion and I don't ever get that with the offset base.
As for the extensions, Woodpecker's agreed to a modification of the extensions included; 2-12" sections each of the sections that extend from 6" to 30" (I think 30") because I really didn't think I would have any use for extensions out to 54" but could have use for doing work up to 30" on either side of the center. So far, I really haven't used the extensions period. I just don't do that kind of work and it's been easier to just mark where I want the Dominos because it doesn't generally matter.
I do use the spacers. There have been situations where I wanted the Dominos centered in the thickness of the wood; sometimes just because I'm a perfectionist and sometimes necessary. It's a handy feature for me. In fact, since the taller spacers are being offered, I may order them this time. I did have a need for one of them awhile back.
I recently acquired the 700XL and will likely buy the adapter this time also.
In general, I think the base itself and the spacers have been very useful for me; the extensions not really. To some degree it depends on the kind of work you do. If you join multiple long pieces, the extensions might be worth it. Depends on how much you do that.
I am a "hobby" woodworker and am not really concerned with efficiency; just satisfaction with my work and as high quality work as I can produce. I'd have to say it is always faster to merely mark the location of the Dominos across the boards to be joined and make sure you exert enough pressure to keep the machine from moving.
It's a cool addition, very accurate, very well machined, and the results are always perfect when I use it. I would say, though, that the results with the Domino alone are generally virtually perfect also. So, it's definitely a choice and not a requirement for high quality work with the Domino. The Domino is my favorite tool ever, regardless.