mainly, the keyword/adgroup/campaign/negs structure needs to be sound, if the underlying PPC is flawed, the optimizer helps, but not that much. auto keyword bid adjustments, for poorly built PPC, is like putting a band aid on a severed limb.
it also helps to structure your campaigns to have sufficient click and sales volume, the more data the optimizer has to work with, the better job it does.
being patient also helps, the more data it collects over time, the better.
if your adgroups are properly segmented, each can have it's own adgroup CPA bid, this is critical. if you mix traffic types within a single adgroup (say bulk tshirts with one-off tshirts), you're handicapping the optimizer. you'd set a single CPA bid for that adgroup, but the AOV varies between these two mixed traffic types, so you'd be under spending on bulk, while overspending on single units (since the adgroup has a single CPA setting, and is based on sales count, not revenue / ROAS).
there are two settings to choose from, Target CPA and Max CPA... if you can be aggressive (meaning the person's credit card behind the spend, understands things go up and down daily), you'll find more success with Target CPA. Max CPA will build a larger buffer, under your CPA cap, to make sure shorter term swings don't exceed your max. doing this means you're telling the Optimizer to over correct for short term dips, so you'll be optimized at a lower sales volume level (relative to Target CPA). chances are, you'll have many campaigns fluctuating, most often not in exact sync with one another - if the spender can tolerate wider swings within a campaign, or within their account overall, that will tend to even out given the other campaigns / adgroups action in their account, then you'll get much farther with Target CPA, than with Max.
not to be redundant, but to stress its importance... if your PPC's foundation is weak (the first point i made above), the Optimizer will improve your efforts from crappy to not-so-crappy. if your PPC foundation is solid, the Optimizer can do a really wonderful job.
never lose site of what it does - it optimizes your keyword level bids, for conversions... this tool does not relieve your PPC team from anything except optimizing keyword level bids. in addition, your conversion tracking must be sound and reliable. your webmaster can wreck your PPC by mucking up conversion code. if you have affiliates, or run name traffic, or have a long funnel, or otherwise have a complicated multi-channel / multi-touch scenario, that can also do a number on this tool (attribution to the campaign underneath is everything). as long as you avoid major sources of interference with conversion tracking and attribution, have sound underlying PPC, good segmentation of adgroups, this tool does work very well.
lastly, it does one more very important thing for you. it frees up time and money. by automating your keyword level bids, assuming you're running your PPC on an ROI/ROAS centric basis, you'll have additional spend and additional work TIME, to build and deploy more PPC. the tool extends your PPC's prime resources, money and time, letting you reach for more volume, with both.