Zuji have been active on Twitter since October 2008 here, so they are a bit of a pioneer - their Australian twitter feed currently has 2,073 followers, which is pretty significant for a branded feed. Their product offering works well in the space, as they have regular e-tail 'events' that are newsworthy to travel minded followers. Who doesn't want to go on a holiday at any given moment?!
I can understand why, leveraging promoted tweets appealed (mumbrella). Their primary motivation would most likely have been to grow their follower base through the ability to appear to a broader base (using twitter promoted tweets/search). Direct actions through to a purchase would be a huge bonus I expect. Actually their primary motivation could have been PR?!
In reality, category based search volumes within the twitter platform are likely to be minuscule when overlaid with Australian filters (eg 8%* regular usage, filtered by the no. who use the twitter search function and then by those who search relevant keywords). With twitter charging on a CPE (cost per engagement - re-tweet, reply, click, favorite etc), sounds like a couple of bucks a day to me for most categories. Having said that, if you try setting up a campaign, the minimum option is $5k to $10k per month (advertising). How much are those CPEs?!
Like Zuji, my over-riding thought is, worth trialing if you already have a considerable social twitter base and are looking for growth. As a keyword strategy competing with more traditional search $, I'm in the not at this stage camp.
*Source: Sensis Social Media Report