I'm a distance runner that's a real 34D - that's the size I wear in regular bras, and this one.  I usually have chafing issues, and this one doesn't much help with that - though the typical problem areas are different from most of the other bras I've tried.

The girls won't move with this one.  It's tricky to get on, and I've got a 50/50 success rate at getting it fastened behind my back by myself (my husband or someone in the locker room has to help the rest of the time, I plan accordingly), I can't get it on pre-fastened (it's just not that stretchy - a GOOD thing).  Once it's on, nothing is going anywhere.  I can't speak much to whether it's particularly cool (I don't care), the fabric is synthetic so eventually it will stink to high heaven, and it it looks enough like a regular bra that a shirt is definitely required over it, but the support is amazing.  I've noticed if I sit or slouch in it, weird parts of the band fold under funny, but since I'm mostly running in it, it's just a symptom of the post-run stretch and doesn't get in my way.

Only down side for me - the shoulder straps are adjustable, and don't quite adjust small enough to what I'm used to feeling on my shoulders.  I don't have impressively perky boobs and they're not naturally high on my torso, so I'm not sure why the straps are so long.  Luckily this bra is solid enough that it's not dependent on the shoulder straps for keeping things in place, so this doesn't stop it from being impressively supportive on me.  I guess I'm used to support coming from the top, not the bottom and sides (as it should be) like with this bra, and it will take getting used to.  It's not like they're loose they just don't leave the grooves I'm used to in my shoulders - perhaps this is just a good thing I'm having trouble adapting too.

Some reviewers have complained about the bra being tight in the arm pits or underwires coming up high on the sides - I think that's just called "support" and most people aren't used to it.  I'd rather have my boobs under control, so I like it.  I would consider myself on the smaller end of requiring this kind of support, though, so unless you're well endowed AND regularly doing very bouncy activities (running, jupming jacks, high impact aerobics), this bra is overkill (yoga, cycling, strength training - don't need this, in fact it might be a hindrance for yoga).

Another note: I don't find my breathing restricted, but - if I were a few pounds heavier (as few as 10) I would (my lungs reach their limit about the time the bra reaches the end of what little stretch it has).  Since I'm the same bra size at my current weight and 10# heavier, I can see why people would order up a size to be better able to breathe.  I'd consider myself a very healthy weight (possibly below a true average), if I were on the upper end of healthy or more, I'd order up a band size (my cup size never changes, but if yours does, maybe keep that in mind).

I used the Champion web site to help me decide on which bra to order, the sizing, and the impact level, so I'd recommend that as well (but this site had cheaper ones).