Doing my Soul Silver Fire type only run, I have taken a small liking to Magcargo. I never really considered it before due to mediocre stats and low speed, despite liking its looks, but with it being the only other fire type than Cyndaquil before the 3rd gym, I grown to love it for what it is.
Most people forget Magcargo due to it only being available before the E4 via the Primo egg, which not many people use because Mareep and Wooper are easy to find. But yawn proved to be a handy utility early on, while having reasonable bulk once evolved when most other fire types are fast and frail, having recover, and amnesia as tools, it can tank even a water attack as well as my other pokemon. Also, rock STAB is pretty neato too, especially when the rest of my team is pure fire.
My original plan was to keep it on the team until I could get Houndour(My sixth Pokemon would have been Rapidash), but now it might stay on the team, or at least be regularly rotated in and out. It did fall behind a little around the 30's as a Slugma, but evolved it has become a pretty menacing (if slow moving) terror.