The latest 13F season is here, when many money managers issue required reports on their holdings. It can be worthwhile to pay attention, as you might get an investment idea or two by seeing what some major investors have been buying and selling.

For example, consider Appaloosa Management, founded by investing giant David Tepper and known for investing in the debt of companies in distress. In a letter to shareholders last year, Tepper noted that had one invested $1 million in his hedge fund in 1993, it would have grown to $149 million over the past 20 years. Investing in the S&P 500 instead would have left you with $5.3 million. Tepper's performance reflects an average annual net gain of 28%. Wow.

Appaloosa Management's latest 13F report shows that it has decreased or eliminated its positions in MGM Resorts International (NYSE:MGM), Trinity Industries Inc (NYSE:TRN), and United Continental Holdings Inc (NASDAQ:UAL).

MGM Resorts International has been at a bit of a competitive disadvantage to its peers, as it has had a bigger exposure to the slowly growing Las Vegas gambling market and a smaller exposure to the fast-growing Macau market. It will soon be doubling its capacity in Macau, though, which bodes well for business. MGM Resorts' first quarter was solid, with revenue rising 12% year over year and revenue from China surging 26%, while costs fell. Overall revenue has been growing steadily in recent years, as has free cash flow, though its bottom line has been in the red. The company carries a lot of debt, but given its growth and cash flow, its leverage seems manageable. Meanwhile, it appears that the Las Vegas market may be turning around, too. MGM Resorts presents an intriguing investment opportunity, but its rivals bear examination, too, as some are growing more rapidly.

Trinity Industries gets most of its earnings from making railcars and is likely to see that business grow thanks to new regulations calling for safer railcars. The company's first quarter was terrific, with revenue and earnings growing 57% and 188%, respectively, year over year. The company's main problem, arguably, is how it will keep up with demand, as its railcar order backlog keeps growing, recently hitting a record level of 42,630 units with a record value of $5.2 billion. Trinity's stock has more than doubled in the past year, and profit margins have been steadily climbing in recent years. With a forward P/E ratio near 12, Trinity Industries' stock seems far from overvalued. Its dividend yield is merely 0.50%, but management recently hiked its payout by 33%, and it has more room to grow. Bulls like Trinity Industries' diversification, as it is also involved in railcar leasing, barge manufacturing, construction services, and more. It's even a major wind tower maker.

The airline industry may have a terrible long-term record, but United Continental has been pleasing shareholders over the past five years, with average annual gains of 56%. Its stock doesn't seem wildly overvalued, either, with a forward P/E near 10 and a price-to-sales ratio below the industry average. Still, all is not rosy with the company, as its last quarter featured a big net loss, and its performance has been lagging that of its peers -- even in customer service. One significant plus for investors is that the company's CEO recently bought $800,000 worth of shares, and owns a stake worth about $20 million, very much aligning his interests with theirs. Though there are reasons to consider adding United Continental to your portfolio, consider its peers, as well, because some of them, such as Delta Air Lines, are more compelling.