If you've never watched NASCAR, you should definitely do it. I've not watched a race in years, but it used to be shown on Channel 5 around 10/11pm, it's perfect 'crack a beer' TV. Very fast, very aggressive, very loud, very full of crashes. The street brawl to F1's refined Queensbury Rules.
However, it is frankly ridiculous in many ways and it's very easy to pick on the fact it's beloved by red necks and so on.
Uhm, I don't have channel 5 because I'm not in the UK nor I do have a tv ;) but I have watched some fragments and I don't buy it due to the oval thing. The crashes, yeah, they are cool, but it's a bit like watching gymnastics hoping that the chick will fall off and crack her arm. It's too bad tourer races (DTM, WTCC) don't get as much attention as F1.
In the "sports I didn't watch" category I never expected motorcycle racing would be more exciting than F1. I've never shared an interest in bikes and I don't know most of the names except Rossi and Pedroza, but once got into watching a MotoGP race and wow, there is action!
It was when you'd mention Top Gear and 'British' and I put two and two together and got 8. Honestly, you should try at least one race purely to see how aggressive some of the drivers can be. It's similar to the British Touring Car Championships, but even less polite.
Superbikes is one of the reasons why I don't watch the F1 as much, it's far more entertaining, the drivers seem to be a lot more humble, and rules aren't constantly being monkeyed.
Rallycross is fantastic to watch but mostly organised chaos, which is no bad thing. Rally is fantastic but I've not seen it properly for a number of years now, it got passed pillar to post in the UK so the coverage was always terrible, so I try catch it when I can.
However, it is frankly ridiculous in many ways and it's very easy to pick on the fact it's beloved by red necks and so on.