My impression is that among Southern Baptists there is relatively little knowledge of the Plymouth Brethren movement. In the country of Spain, where my wife and I are missionaries, though, they are one of the main evangelical groups. I would say "denominations," except for the fact that many "brethren" profess to be opposed to denominations, and do not recognize the denominational character of the movement with which they identify.
For the most part, Plymouth Brethren are conservative evangelicals, even more conservative on several points than most Southern Baptists. I have had the privilege of sharing some wonderful, warm fellowship in the Lord with various individuals and congregations associated with the Plymouth Brethren movement.
Like various other Christian groups, the Plymouth Brethren started by and large as an attempt by sincerely motivated believers to work towards Christian unity, and overcome the barriers of denominationalism. Sadly, though, in many cases, the practical outworking of the application of the principles behind the movement has led to increased sectarianism, similar, in some aspects, to the Campbellite/Church of Christ movement, and the Witness Lee "local church" movement.
In any case, I think there are many things we can learn from the Brethren, including both lessons from their achievements and shortcomings as a movement, as well as some excellent biblical reflection and spiritual exhortation. Several names associated with the Plymouth Brethren who have made an impact on evangelicalism at large include George Müller, J. N. Darby, Harry Ironside, Jim Elliot, F. F. Bruce, and Luis Palau.
This past Christmas, Dec. 25, 2007, William MacDonald, one of the most beloved and respected Bible teachers associated with the Brethren movement, went to be with the Lord at the age of 90. Like so many other godly leaders down through history, MacDonald wrestled with the practical implications of biblical teaching on Christian unity. Although, in the context of what he writes, MacDonald specifically addresses some of the quirks and idiosyncrasies within the Brethren movement, I think the following article contains some very rich food for thought all of us as Bible-believing Christians, and specifically, as Southern Baptists.
Read To What Should We Be Loyal, by William MacDonald