I am my agent's first client (her list has since expanded to about 9 total). She's with Talcott-Notch (Jessica Negrón), and I had originally queried Gina, but Jes fell in love with my book and pulled it out of the slushpile. After an R&R with Gina, Jes was promoted to junior agent and requested the opportunity to offer me representation. I accepted because A: I trust Jes's passion for both my book and her career path; B: she is sure this is what she wants to do and is under the mentorship of Gina as well as the other experienced agents at Talcott; C: I knew I wanted an agent who was looking to represent me for my full career and interested in everything I want to write. Those reasons among many, many others.
In the nearly 12 months I've been her client, she's gotten my books in front of phenomenal editors. With the first MS there were several near misses (it got to acquisitions several times and garnered a bunch of lengthy and very complimentary pass letters from the editors), and once a book gets to editors, it's the work, not the agent that makes it power through the acquisitions process. I don't regret my decision for a second. The relationships we formed with the first editors are a great foundation for both Jes's development as an agent and for my career. There are people who are excited to see my work now, and the new MS we subbed in July got a full ballot of positive first responses (meaning every editor she sent a proposal to wanted to read it). That's how I've judged this first year. She's also made sure her process is transparent, and she emails me updates every Friday.
New agents can be awesome (mine is), but it's very important that they come with experience (some come from working as an editor or some other branch of publishing), reputable mentors/agencies at their backs, or both. The important thing is to vet them as you would anyone else you planned to enter into a business relationship with. If they have clients, contact them. Ask about submission processes, communication, whether the agent is an editorial agent or not, and whatever else matters to you. If something doesn't sit right, trust your gut. No agent is better than a bad agent. Repeat that until you believe it; it's true. If the agent you're researching has sales, see what they are and to whom they sold. If they don't have sales and offer you rep, ask where they plan to submit and who will be helping them along the way. Any agent offering you representation ought to have at least an idea of which editors would be interested, even if they haven't been around enough to cultivate personal connections with the editors yet.
Having several friends who have recently terminated agent relationships for various reasons (with experienced, reputable agents), it's important also to note that experience does not make an agent the right fit for you. Regardless of someone's level of experience, if you're offered representation, make sure you ask candid questions to clarify all possible expectations of your relationship. How you'll communicate, how much you will know about your submission process (basically, if you ask them anything, they should be willing and happy to answer Qs about your subs) and when you'll hear about it, etc.
Anyhoo...just my two cents. "New" doesn't preclude greatness, and as with any other agent offering you rep, you should do your homework to the fullest possible extent.