I'm a recent law school graduate who didn't summer at Cahill, so take this with a heavy grain of salt, but from what I hear from a close friend who summered there on the corporate side...
It's just more specialized than other firms. The firm has tremendous strengths in acquisition finance and the capital markets, particularly in debt. If you'd like to do lender- or underwriter-side work, wonderful, you'll be at a firm with an incredibly healthy book of business doing term loans and revolvers and everything else. They do other corporate things, too, but the firm has a strong point and the associates know it. Really nice compensation and tech stipend, though.
The other firms have their own strong points, though as a rough impression they seem to be a little more diverse than Cahill's practice strengths. Willkie has an M&A practice that does just fine, with some pharmaceutical companies and PE firms. Dechert has investment management and real estate strengths. Milbank's also a finance-based firm, but has project finance, other specialized finance, bankruptcy, etc. Fried Frank has a lot of old M&A clients plus its foothold in the real estate market. There are plenty more differences between all these firms, as I'm blatantly ignoring culture, how you might pick up work, and so on. But every one of them's just as respectable as the others.
If there's anyone here who has actually worked at Cahill, please fact check me.