I have a fairly large corpus of events with spatial data which are indexed in Solr. LatLonType won't cut it for me. I'm a huge fan of David Smiley's work with SOLR-2155 which I used to scale my app in Solr 3. David's embraced spatial grid implementation and I've found his modules outperform LatLonType in my use cases - other benchmarks support this.
Where SOLR-2155 required an additional JAR, Solr 4.0 ships with a solid geospatial solution. I've been using solr.SpatialRecursivePrefixTreeFieldType with pretty much standard options.
There was a lot of weirdness in Solr 3 which is now fixed. Namely, bounding boxes were actually circles - now we get simple bounding rectangles. In fact, polygons are within our reach. If you don't mind some LGPL in your app, you can include JTS and get it with Solr 4.
I don't have metrics to quantify performance yet. Once I get more data indexed, I'll try to post some numbers. Thanks David!