As part of nomitor's setup we've been poking around at a few CDNs and hope to eventually publish a summary of our experiences. At this stage we haven't got quite enough data to do so but stumbling across Mike Brittain's Tips on Selecting a CDN Vendor I thought I'd add a few comments here as he has disabled comments on that post.
Keep in mind this is from the perspective of a very small startup with absolutely no buying power so I hope this is useful to other small companies who may be after basic HTTP delivery (i.e. serving static objects like JS/CSS/images, as opposed to streaming video).
Besides price, features such as geographical footprint, gzip and the ability to set appropriate cache control headers are likely to be key factors in CDN vendor selection. Not every business needs global coverage and there are some CDNs which only cover US & Europe which may be more economical. Some CDN providers explicitly charge more for Asia coverage (including Oceania).
Points 6 & 7, regarding the support model & transparency of status. If you're a very small customer you're more than likely going to end up buying CDN services through a reseller as very few CDNs will sell direct at small volume. In this case you need to understand exactly how support issues will be escalated - are you able to open tickets directly with the underlying CDN vendor, or do you have to escalate through your reseller's NOC?
If you follow point 4 and compare speeds from various locations you will more than likely discover irregularities in every CDN network. You might want to use that data to open a conversation with the CDN provider to establish how responsive their NOC/support is as you don't want to find out during a real outage that the promised support is non-responsive.
In actual experience we discovered regional nodes that were down for hours (and not failed out of rotation), as well as nodes with consistently poor performance, and even nodes that were "in production" but never in rotation (resulting in no local presence for that region). I won't name names as these may well have been corrected but I should say that as we're currently using EdgeCast (via a reseller) that these issues were *not* seen on EdgeCast.
The good news for small CDN customers is that there's an increasing number of CDN services available on a PAYG basis. EdgeCast is available via SpeedyRails, Media Temple, and GoGrid, HighWinds via VPS.NET, and LimeLight Akamai via RackSpace Cloud (though this is lacking Origin Pull). This is not meant to be an exhaustive list, nor any sort of endorsement, but just a few off the top of my head that have published PAYG pricing. You can find a complete list here but keep in mind that a significant number of CDN providers focus on delivering video and it can be quite an arduous process to whittle down suitable options; CDN Planet may be a simpler way to look for a CDN.
Update (4th May 2010): VPS.NET is now also reselling Akamai at very competitive rates.
Update (3rd Nov 2010): SpeedyRails is also reselling EdgeCast
Update (12 Nov 2010): AWS CloudFront recently added custom origin & gzip support making them a strong contender. The only remaining weakness is a lack of presence in Australia & NZ; from Sydney I land on POPs in Singapore and Tokyo which is not ideal. Hopefully this will change over time.
Update (22 Mar 2011): Media Temple resell EdgeCast too.
Update (12 Jan 2013): SoftLayer also resell EdgeCast, they moved away from Internap in April 2012.
