I think it depends on the nature of your writing (e.g. niche market vs mass, fiction vs non fiction) and your location. For example, I'm in NZ and it is very common for publishers to deal direct with authors. We also have a great author's society here that gives a lot of advice on contracts etc etc. I did deal direct with a US publisher on an anthology and that was fine. We argued a bit on the terms and in the end came to a middle ground I was happy with.
I also had a deal with a canadian publisher but they never published the book. I wasted a lot of time and ended up having to chase them to get a release from the contract. My point is that there's no crystal ball. You may get a good deal and things may go fine or your ms may get tied up for a couple of years and waste a lot of time.
it could be worth having a reputable agent to look after your best interests. In theory they earn back the cost of the royalties by getting a better deal than you would on your own. I guess I would think of it as insurance. It costs you and you may not gain anything at all, but the potential upside/downside is huge.
If the ms is "hot" they may find a better publisher for your book. Plus agents can give you a lot of contractual advice, such as whether to give the publisher options on future works, under what circumstances etc etc.