Is it worth it to go for rich minerals?

The value of gold mainly depends on how well you can defend it. The advantage of mining gold minerals is pretty significant, but most gold patches are in positions that are extremely difficult to defend, usually because they're in large open areas with multiple avenues of approach, and also because they're very far from the starting base positions. However, from what I've seen of pros playing, if you have enough of an advantage to secure a gold base, you should probably just be pushing for the win--but it can certainly help you finish a close game that's stalling out.

If your opponents are trying to grab gold immediately, it probably means they're overestimating its value (especially over early mining and tech) or are expecting you to not be scouting well enough to find it. Either way you can heavily punish that decision by going aggressive with your unit production and attacking them before they can get much built.

It should be noted that moving one's first Command Center to a gold base is never worth it.

EDIT: There seems to be some debate about the efficacy of the so-called "Gangnam" strategy of lifting your CC and moving to the gold minerals, especially on Prion Terraces which now features a defensible, natural gold base. It seems that builds designed for this purpose can bring a player to an advantage, if they don't face any early rushes.

Generally, you have to take into account two factors when deciding whether to take a gold base or a regular one.

First, is the map itself. As others have pointed out, they are usually in places that are difficult to get to or defend. In this case, you have to evaluate how likely you are to be attacked at that location before you are able to defend it, and whether the location is more of an asset or a liability.

There is a map in the current map pool, Prion Terraces, that features high-yield minerals at both the third and fourth expansions. Many players opt to grab the high yield as their first expansion, as it isn't that far away from the main base. It is a bit more difficult to defend, but not impossible, and holding the forward position gives a player a little more map control (as it is more difficult to contain players on the low ground) and makes taking the third base (the natural, now safely in your pocket) really easy.

The other thing to take into account is your strategy in the match. The fact is that you will be bringing in minerals noticeably faster, and noticeably faster than you are bringing in gas. Therefore, you are going to want an army composition that features more mineral-heavy units. If you risk something to take a gold base, and end up floating alot of minerals that you can't seem to use, then it was a wasted effort.

However, if taken at the right point in the game it can pay off handsomely. In last year's GSL, a lot of Zerg players were catching Terrans off guard by sneaking an early gold base on Vaani Research station, then quickly pumping out zerglings and roaches and overwhelming their opponent who could not keep up in production. At first, it looked unbeatable, but Terrans soon began to scout it and counter it and the strategy would sometimes fall flat.

A gold base can supercharge your strategy if taken at the right time and leveraged properly. However, bear in mind that it is often a risky move as it can divide both your army and attention at a critical point in the game.

To be honest if something like a mineral grab is going on your opponent wont know how to counter a zerg/zealot/marine all in since they have wasted about 3 min just moving there base to that position.