A complex, advanced automation platform that's also easily customizable and usable

It's expensive, so obviously not meant for smaller SMEs. Ideal for automation (obviously) in terms of nurture programs and scenario identification, but not a full CRM (i.e. once a lead is responded to, you also need to have a Salesforce-type CRM which can be used to assign leads) rather than within marketo itself. It's not smart enough to run heuristic identification, so you have to be very exact - eg. USA and U.S.A would be considered different things. Be prepared to do some really, really deep and extensive preplanning before setup to avoid messing up your data - it's immensely powerful, so a high risk of carelessness having very far-reaching consequences.

There's a fair amount of flexibility, but as you start moving beyond the basics and get into more complex programs, it gets a bit cumbersome; for example, flexible drip campaigns that can customize / change content based on triggers after the initial start. Some way of normalizing scoring so older, less intensive users don't overshadow newer, more-intense users. Etc.
For a basic setup and taking control of the user flow from contact to lead without leaks, Marketo is awesome, once you've got the core data governance and standardized processes in place.
Template design is difficult - designers find it restrictive and there is a LOT of debugging needed, with finished templates sometimes behaving very oddly in some environments - adding / deleting code snippets, disabling some scripts - not enough to break it, but definitely some odd behaviors.