In my extremely limited experience (just the one house!) they're brilliant. Can't think how else you can run lots of services without using a suspended ceiling. And you can carry a >5m span up a ladder with one hand! And your floor finishes up true. But watch out for the webs lining up if you've a run of different lengths.

I was speaking to a joist supplier who sells both recently and got the impression that Posi joists cost about 10% more than timber I-joists. I am probably going to go with Posi joists for my place for the ease of running the services. My architect has used them several times and they are a pretty standard component these days.

As said above, if you are using different lengths then you need to be careful about getting the webs to line up. Also think about how you are going to fit the MHRV ducts. Depending on what sort of ducting you are using, will you be able to insert the ducting after all the joists have been installed or do you need to insert some of it as the joists are going in? If you are using long bits of solid ducting then inserting it after the event might be a bit tricky! This has been covered here:

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=5461&page=1

If you are using any recessed lighting then think about the sound implications of open web. I have no data on this but intuitively open web joists seem like they ought to let the sound travel more than timber I joists with just a few holes cut in them.

A and A above,
Thanks for your input....It looks like Posi Joists are the way to go for us, but i see what you mean about running the ducts (rigid) in them first....

Cheers, and thanks again