RIPE Labs posted an article a while back, titled Is Is Really Worth Peering at IXPs? A Comparative Study.
The main comparison is between data flowing over a path with traverses an IXP, versus a transit path with uses commercial ISPs only.
The authors conclude:
Our experiments put in evidence that peering arrangements that make use of IXPs have a positive effect on key performance indicators such as latency, hop count, packet loss and jitter. They also reduce the number of foreign ISPs traversed by the traffic between users located in Italy and critical Internet services like Banks and Public Administrations.
Ready the study for full details, but rest assured that IXPs perform well: They shorten network paths, which increases the performance of their members’ networks, plus keep traffic domestic.
IXPs are good for the internet, let’s keep building them!