I've had (cough, cough) several agents now and have done some deals by myself and some agented. I think the biggest bonuses for me when it comes to having an agent is not money-based (which is what most people think), but a) access to publishers/editors and b) having someone as a buffer between me and my publisher/editor.
With an agent, you have access to a far greater range of publishers and editors than you would otherwise. Agents also are able to keep track a little better of who's moving where, who's on maternity leave and so on (people move around a lot in publishing). Also, sometimes publishers/editors will specifically approach agents asking for submissions. Just last week I pitched something to a publisher through my agent as the publisher was looking for something specific (this is in the US, though -- if you're interested in the US market, you'd definitely need an agent).
The buffering I mentioned has been particularly helpful in a couple situations in my career. Like when 50 books turned up on my doorstep in another language and I slowly but surely worked out it wasn't my book, but someone else's book with my name on it (the publisher had gotten the title and author mixed up). My agent dealt with that. Or when my publisher (and I'm not being specific to my current publisher here -- I've had a few!) type-set the uncorrected proof of my manuscript, didn't want to re-type-set and tried to fob me off about doing so. Again, my agent dealt with that. Thank goodness, because it could have become pretty embarrassing.
I can't think of one downside to having my current agent. She is simply amazing and worth every cent. However the downsides to my other agents have been lack of communication, not wanting to shop some of my projects and generally holding things up. That said, some deals are pretty set in stone. Harlequin Mills & Boon, for example, has boilerplate contracts. There's not a lot of room to manoeuvre in their contracts and so lots of H M&B authors don't have agents, which makes sense. If you have access to the publishers/editors you need to access right now, I'd go it alone for a while and see how things pan out, or approach agents at the same time as you're approaching publishers.
Hope that makes some sense!