“community seems to be quite small”: Tiki community is huge. Just pop by on IRC to meet some people.
“learning curve for me will be huge as the documentation is er... well somewhat lacking.” : About documentation & the learning curve. I suggest you download and print out the 350 page Tiki 1.6 documentation in PDF. While many more features have been added since, it is an excellent introduction document.
“a complex site up and tailored to our needs (including the look and feel)”: Tiki is an excellent solution for a complex site as it has many features and many options for each features. And then, you can customize the look & feel via the excellent Smarty-powered templates. Tiki has many smart features to manage large sites, including the site-wide search, the theme control center and the category system.
“That much time with Drupal or e107 seemed to take me much further than with TW.” : hmmm, I’m sorry to hear that. I have not looked at them for a while, so I can’t comment. Please see: WhyUseTiki