Personally, I think that IQ tests can be useful, but they do have to be taken with a very large grain of salt. IIRC, IQ does tend to correlate quite strongly with a number of interesting measures - level of education attained, annual income etc. On top of that, most 'online IQ tests' are really not. Many have similarities to IQ tests, but aren't necessarily accurate. Most don't take into account your age, which a well conducted IQ test should do. And some are just awful, asking trivia questions and the like, which is just crazy.
They're far from perfect, however. They measure your ability in specific skills - things like pattern recognition, some stuff with language and reasoning etc. But a 45 minute test (or whatever) isn't going to give a perfect value for how intelligent you are, that's just not viable. Too many people take them too seriously, using their IQ as a bragging point or to try and denigrate others (I used to do this, as a kid ¬_¬), or else feel ashamed of their low I, when really they could have intelligence in other forms. Conversely many people write them off as worthless - until recently I did this as well, saying the only thing IQ tests are good for is measuring how good you are at IQ tests. Well, that's a bit extreme in the other direction. They do give a decent indication of intelligence, at least, even with their flaws.
Anyway, that's just my view on the topic.
The last time I took an IQ test was over a decade ago. My IQ was 30-something. I can't remember my exact score because I'm not very bright.
You're probably misremembering this. Like, somewhere around 70 IQ is the point you're classified as having learning difficulties, IIRC, and around 50 or below is considered pretty severe. An IQ of 30 would be pretty incredible, in a bad way.