We have been running solr in production for 6 years or so. But I have with interest been following the ES development blog, since I think they have a nice api/features.
The momentum/community behind elasticsearch seems to be building rapidly(to get an idea, just follow the "This week in Elasticsearch" posts on http://www.elasticsearch.org/blog).
Having said that, solr have been rock solid for us so we have no real pressing need to switch. Also we dont really have the problem(massive scaling) that elasticsearch seems to be built to handle, we just have 15 mio pageviews or so pr month. Our setup is 1 solr master, and 5 solr slaves(one on each of our 5 webservers). And do nightly dataimports from our SQL server database. That last part is indeed where I think solr currently have a nice advantage over elasticsearch. The solr dataimporthandler is really nice if your primary datastore is a SQL server, and allows you to do all sorts of nifty javascript and other transforms on the data in-flight as you stream it from your SQL server. For elastichsearch there is a jdbc-river thingy that sorta lets you do the same, but it isnt as polished or usable as the solr dataimporthandler(IMO). And if you want to install it you have to do it via a plugin link that points to a bit.ly address.. which makes me feel uneasy.
I also like that solr comes with an admin GUI out of the box. There exist some ES equivalent plugins(mobz/elasticsearch-head), but like with the jdbc river its a thirdparty plugin and I guess you have to trust that it doesnt screw with your server. With solr all you need comes with the distribution, so you dont need to spend mental energy on wether or not you can trust this or that plugin to run on your server.
Also the .NET client for ES seems very polished and more sexy than the solr equivalent.
Anyway my non-scientific gut feeling is that with the current momentum behind ES, it will over time be a better choice than solr. But unless you really need to scale massively, plain old solr is available and works just fine. And seems to me to be somewhat easier to get running than ES(but then again I'm probably biased after having run solr a long time).
