I have a few so here are some thoughts.
SHAPE:
Round: I love the shape for gatherings but it takes up more space overall than a rectangular.
Rectangular - depending on how wide it is, it can be hard to serve family style, there just isn't that much room down the center.
Square - LOVE - but if you have an odd number of people it looks off balance and bothers me.

Mine extends to a large rectangle and the seems are very tight so I've never had any issues with things getting stuck. It also takes up more space than even a round table. But it is way cool.
FINISH:
Wood - no matter what, something is going to happen to it. Our dining room is cherry and distressed. So I don't worry about stuff happening to it. However, the finish is awesome and hardly anything bothers it. But we had to
pay for that finish, it is from Restoration Hardware. If you for for a super shiny formal finish it is going to show stuff unless you use a tablecloth.
Wood w/glass topper - this is what I have in the kitchen and I love it. You don't get prints and such under the glass, just on top and I have to wipe it down after each meal anyways. The glass will scratch though. But I figure replacing the glass in a few years is much cheaper than a new table.
Wood Veneer - DON'T go there. My big square-to-rectangle table is a wood veneer on top and I hate it. It is gorgeous but I have to totally baby it. I H.A.V.E. to use a tablecloth every single time. I have a bowl that sits on the table as decoration and I had to put felt feet on it because it was scratching the finish. Love the shape of the table, hate the fact that it is veneer.