I'd assume geoapi would be reliable (bc it's twitter) but not sure of any twitter-sized sites using simplegeo. Thanks.
I can, however, describe some of the technology that we use.
We've built the platform using a multi-homed, multi-master database solution we call GiselleDB. It's based on Facebook's Cassandra, but we've forked it since then. Basically we're able to handle lots of read volume, lots of write volume, as well as push the data to all of our data centers. We're currently housed in three datacenters and will expand to more later. Essentially this means we can handle plenty of volume and it's always redundant (with API failover if a datacenter goes down).
I'd encourage you to get ahold of me (matt [at] simplegeo [dot] com) or our developer relations guy (andrew [at] simplegeo [dot] com) if we can answer some further questions for you.
SimpleGeo does provide "writeable layers". This is our primary feature. Essentially, developers can create a private layer, then start populating that layer with points. Those points can be checkins, media, breadcrumbs, points of interest...anything, really.
The SpotRank data (provided by Skyhook) is an additional API endpoint that allows you to see the current population density of a given area based on cell phone usage.
We do provide breadcrumbing, or user history if you will. This pairs nicely with iPhone OS 4.0's new background location.
Beyond all of this, we're turning on our Marketplace shortly. We've gone out and found lots of great geodata to expose to developers. Some developers have also begun to contribute their own geodata to the Marketplace.
Hope this helps! (We do need some better messaging on our website for sure).
We didn't look too hard at SimpleGeo but I can say without hesitation that GeoAPI has been fast, stable, cost-effective and easy to use. In the rare cases where we experienced issues they were quick to respond and resolve them immediately. I also know for a fact that they are servicing some very large customers as well (larger than us for sure :).
GeoAPI gives you access to a pretty rich dataset for the United States off-the-shelf, which was very important for us.
GeoAPI
+ Got us an API key within 5 minutes of requesting it
+ Comes with a comprehensive list of points of interest, from place data (restaurants, bars, ...) to other (parks, cities, etc...) for the US. This was the single most important factor as it enabled us to build a place-aware application without worrying about places too much
+ Has an API that scales and easily handled our bump in traffic due to being TechCrunched
+ Has extremely supportive and fast technical support and issue resolution. The engineers, despite being at Twitter, still react in "startup-time" which is critical to our success
SimpleGeo
- Took 2 months to get us an API key
- Still has no POI data. This is apparently coming in their marketplace but will cost an extra fee and there's no way to test the quality of the data to-date
Basically, it was no contest. I have no doubts that the SimpleGeo guys have built a great way to index & search for data spacially, but without a comprehensive POI database, the effort required to build a geo app is much greater. With GeoAPI, we not only got fast geospatial search but also a well organized list of POI to go with it.
Hope this helps.