Hi Alexander -
If you want to check out real stats without the hype from individuals' own sites, look on http://www.FranklyMLS.com/pt
where you can pull up all of an agent's history (pulls both listings and when they helped buyers). The site has won numerous awards and was built buy the founder, a tech guy, to be user friendly and give as much info as any buyer or seller could want without the typical "let me sell you" slant. You may also like the blog if your interested in hearing about a different approach to buying here in Northern VA. DISCLAIMER: I am actually a Frankly Real Estate agent myself.
To do the agent research on http://www.franklymls.com/pt
follow these steps: type agent's full name into search field (ex: "Michael Rosen" instead of Mike Rosen), then select from the drop down boxes "No Min", "No Max" for pricing, and for Show select "Sold 2009+" if you want more recent activity. You'll then be given a spreadsheet with all the buyer and listing sales for that agent (you can sort using the headers). The spreadsheet defaults to 30 properties per page, but you can adjust that at the bottom. It will show you days on market, tax assessments, list price, sold price, and many other fields that you will find helpful in researching an agent's track record.
Let me know if you have any questions on this feature.