I get the impression many keyboards billed as "semi-weighted" are either weighted or synth action. Curious to hear from people who have or have played semi-weighted workstations (eg Roland FA07 etc) just how weighted they really felt...stores here offhand aren't carrying any.
Typically, weighted actions have hammer mechanisms, that's what makes them so different from "semi-weighted" actions, not actually additonal weight per se. Unweighted ("synth") and semi-weighted actions are broadly similar to each other, except weight is added to the semi-weighted action compared to the unweghted. (A more thorough discussion of this is at http://forums.musicplayer.com/ubbthreads.php/topics/2908887/Semi_weighted_keyboards
) So when you ask "just how weighted" a semi-weigted action feels, don't expect it to feel anything lke a "fully weighted" action... not because of less weight, but because of the lack of the hammer mechanism.
Now, if your underlying question is, how amenable is a given semi-weighted action to piano playing, that's really a different question, as that is not necessaily defined by how "weighted" a semi-weighted acion is. In fact, the Yamaha NP-30 series is unweighted, but feels beter for piano than some semi-weighted actions, IMO.