I use IndexTank for my startup(pilarhq.com), and search is the single most important thing for us. Seriously, IndexTank is terrific. Not only is their product pretty fast and provides great search which you can integrate in your app in a matter of minutes, but their support is AMAZING.
In fact, I've tweeted TWICE about how great their support is. They've seriously walked me through each step with various code samples and have resolved all my issues within a couple of hours....I have only great things to say about them.
We've (Gazaro - http://www.gazaro.com) started to use IndexTank recently and I have nothing but good things to say about Diego and his staff. Their service is super easy to work with, it's fast and reliable. We've had a couple of problems but they've been there helping us every step of the way.
Search is not easy (or at the very least time consuming to implement properly) so if you're looking for a fast scalable solution I'd recommend having a look at IndexTank.
The reason why IndexTank is probably so palatable to people is that it's somewhat of an arcane art to setup and scale Solr. There are other reasons to outsource search (and the Wordpress plugin looks pretty sweet for non-techies), but if dread towards configuring a search engine is the only thing that's holding your company back from keeping search in-house, you'd do well to take a look at ElasticSearch. Lucene-based like Solr, but set-and-forget for most purposes.
In fact, I've tweeted TWICE about how great their support is. They've seriously walked me through each step with various code samples and have resolved all my issues within a couple of hours....I have only great things to say about them.