Downsides you've heard: bulky (one mold pretty much, with different control penetrations), some of the controls can be a little awkward (same reason), heavy (ditto), viewfinder isn't ideal (I use live-view with my camera). To that you might add that some people disdain the iTTL built into the housing, "locking you in" to their strobes if you want TTL, and costing the money even if you didn't.
The other usual complaint is related to the port attach with the clips, which doesn't seem as robust as it is. (It's more a perception of failure due to a tendancy to 'rock' at the joint when under no external pressure. I can convince myself it's actually fine if I think about the mechanics, but it can scare me just the same.) Lastly some purists will make a big deal about glass vs. acrylic for the dome surface...purely a cost trade there, and while acrylic can scratch easier, at least it can be polished out at the end user level unlike if glass does scratch.
My biggest current complaint is I wish it had a top mount for focus lights or a 3rd strobe. I make do with a 3-way ball joint off one of my arms, but that means everytime I adjust that strobe I have to re-aim the focus light too, which is a little annoying.
Upsides: Come with handles (not all housings - Olympus for example - do, although some are designed for use without them), comes with iTTL (I think it's a plus vs. having to buy an additional adaptor as I do use Ike strobes right now, and I like having the control down on the housing vs. having to fiddle with the strobe heads). The larger bulk means all controls are utilized (maybe not for all models, but certainly 'space contraints' isn't a reason to leave one out terribly often). Clear is nice to 'see' seals and potential leaks, but not as huge a deal as I think they make it out to be (only the exterior face of the seal is clearly visible). Very sturdy both controls, hinge brackets, etc. Does not scream "steal me!" with its custom milled shiny aluminum goodness. Excellent customer support. Significant price reduction vs. more customized housings. I believe (but do not know for a fact, as the weight of the housing itself due to wall thickness is certainly a factor here as well) that they tend to be a bit less negative because of the internal volume vs. more form-fitted housings, reducing (NOT eliminating) your search for flotation for compensation. Big enough customer base that if you do want to change you'll get rid of leftover ports and domes in a hurry....although this is a tight-knit enough community the other ports likely change hands quite easily too. And while I put the 'clips' down as a con for some people, I also think in some ways its a pro: I am not a fan of threaded or bayonet mounts for large diameter openings, as I don't think you know you haven't pinched an o-ring as easily. Of course Ike's extension-to-dome mounts are threaded, so.... 
Disclaimer: I am a first time, still new DSLR user underwater, and the Ike housing is the only one I've owned. Prior experience was with a Canon OEM housing for a little PnS.