Hi Paul,
I actually came here looking for the same info. I'm half way into my 30-day trial, and I have to admit I'm pretty impressed. I get prospect e-mails with the company name, location, pageviews, and referrer of the visitor. That would allow me to send marketing materials to that visitor knowing how they got to my site and what they were looking for. Really cool!
There are a lot of other great tools, like scheduling posts on social media sites (think Hootsuite), form builder (think Wufoo), a simple CRM (think Highrise), keyword analytics (geographically limited and not nearly as detailed as SEOmoz), other SEO tools for content and blogging, and the most awesome feature which is e-mail marketing automation (think Salesforce.com workflow for e-mail). This e-mail automation allows you to set up a flow to "nurture" leads into customers, so if a certain link is clicked or customer information is entered on a certain form or landing page, Hubspot knows and starts sending the e-mails you have created on the schedule you set up for that specific call-to-action. Point is to send pertinent information to leads base on their activity on your site. Now, for a niche business like American BBQ catering in Taiwan, this is really cool. It's probably not so useful for standard e-commerce sites or low-cost, high-volume businesses.
On the down side, and I mean really down side, is the native content management system. Sorry Hubspot, is just sucks. Just a little bit more love on that side and there wouldn't be too many services that could compete. And pricing is an issue for us. The version that would allow me to use the Hubspot API on an external site is over $8K a year, which even if my lead conversion increased 32% (the number they boast), then I'm paying over $200 for every new customer. Those numbers aren't very attractive to me, but they might be for you.
Great people there, great support, intuitive system, terrible CMS. But at the end of the day, the system could help you save time by merging lots of systems into one.
B+
Kevin