Really? I have one of each (Supermicro 5-in-3 and i-Star 5-in-3, same ones you have, I think) and I think the i-Star ones... suck. Apart from being shallower and a bit quieter (not really an issue in the case I have them in) they're cheaply built, the drive caddies fit the cage poorly, the releases are annoying to use, the "locks" are damn near impossible, and they just don't feel sturdy.
I guess the noise is probably more important to you, though, given that one of your goals was "quiet" - the case I have them in is anything but so the noise the cage makes doesn't really matter to me.
They are a lot quieter and you have the option of setting air flow, they also fit with a better tolerance (anyone who tried to fit 4 of these in a stacker vs the istar ones knows what I'm talking about). Also, the cable positionings are better and the power connections are better as you have a little bit more options when wiring.
Aside from that, quality wise the Supermicro is a ton better, but for the added weight (a lot heavier), noise (a lot more noisier) and some of the other things as mentioned before... not to mention price, the istar is a much better unit. Oh, you also have integrated signaling wires, so no bundle of wires or trying to figure out where to plug what in (not to mention, it helps with wiring jobbies and it's painless when dealing with cards which may not have the correct signaling headers).
Also, it may be worth mentioning that you can switch individual drives off, reset, and shut down the whole unit using the buttons on the front of the unit... oh and you can lock it... makes it very functional.
So, your using the fans that came with the HD enclosures? Are they quiet enough for you? Early on i know you wanted to make 4.0 as silent as possible and you got fans that (i thought) were replacements for the HD enclosures.
Is this what your using - ISTAR BPU-350SATA (if not can you post the brand/model again)? Also, did you (or anyone else) try replacing the fans in the Supermicro 5x3 enclosures? I found the same problem looking for 90mm, but do they have to be 'thin' like yours need to be? if not, it may be worth looking again for silent 90mm's for the easier swapout
Yes, i am still using the same fans that came with them. The unit has an option of Low Meduim or High settings. On the high, it's as loud as the Supermicro unit (maybe still a little less), but on low, it's reasonable. It's not as silent as I want it to be, but then again, not much I can do about it.
Yes, you can replace the fans on the Supermicro unit and they are a bit easier to find (nothing really special about them).