It's not the log storage that we care about, it's what we can do with those logs.
I've been through three logging vendors in the last two years (Loggly, LogEntries, now Sumo Logic) and I'm looking to move once again. The secret sauce is not the storage volume; our volume is relatively small: 4.3 GB/day. The secret sauce is the intelligent structuring of unstructured log data, analytics+alerting on the now-structured data, and live tail'ing of logs.
Sumo Logic does an okay job at the parsing/querying/alerting/analytics piece but live tailing is a brand-new feature for them and it has limitations. Sumo Logic's pricing is also frustrating--they don't apply volume discounting until you're pushing something like 500 GB of logs each month. Again, we need affordable analytics; storage is a small piece of the puzzle.
Honestly, our next move may be to an internal "HEK" stack: Heka, Elasticsearch, and Kibana. I've looked at the cost of doing this and it's actually more expensive when you consider what we'd spend to host a big ES cluster internally, but cost is not our #1 consideration, usability is.