I have a small position in Transcanada, but I bought it a number of months ago when it was at a low because it provides a good dividend, and the bad news about Keystone getting rejected had suppresed the value.
Transcanada has a huge and diversified portfolio of mainly natural gas pipelines, and other projects such as oil pipelines, powerplants, etc. I bought because I work in the industry and I know they have been dilligently building and aquiring a huge network of pipelines in the background that never received publicity because they were natural gas instead of oil pipelines.
Bottom line is while I am sure Keystone XL may be good for share value, I would have bought the company anyways and I know it wont be the single determining factor in their long term profitability. They are a good solid company that has been around for a very long time and it will be around for a very long time and continue to pay dividends with or without Keystone XL.
Finally, you might earn some good money off Keystone, but I doubt you will get exceptional multiples, so I wouldn;t sink your fortune into a single position. Its a boring (in a good way) dividend stock and I personally will consider myself lucky if it does too much better than the market index.