My recommendation is to use only what you need to use, for what is in your tap water.
Except for sik80 who is in the UK, the rest of us who have so far posted in this thread are in North America. That makes a difference. Municipal water in NA will almost certainly have chlorine in it. In the UK many places don't, or it is so minimal that many highly-experienced aquarists do not use conditioners. Liverpool might be one place that does use it, being a major city.
Chlorine itself is easily removed by various methods. However, many municipalities in NA now also use chloramine, which can only be removed by chemicals. Most--but certainly not all--water conditioners handle both.
The next thing to look for is heavy metals. Most, but again not all, conditioners detoxify these. Those of us with planted tanks wish they did not, because the level of heavy metals in tap water will be trace amounts and these are valuable plant nutrients. Iron, copper, zinc, manganese and nickel. By not using a conditioner that detoxifies heavy metals you can probably get by without plant fertilizer, which is a money saver.
However, on well water, this could be an issue, if excessive iron, copper, etc is present; depends upon the well water source. Plants can take up heavy metals and detoxify them just as easily as any water conditioner--and in almost identical amounts--in addition to using some of them as nutrients.
Then we have various types of nitrogen: ammonia, nitrite and nitrate. Each one or more of these can be in municipal water supplies. If one of them is, you probably should use a conditioner that also targets this. Few do all of this. Some will detoxify ammonia, a couple will do ammonia and nitrite (Ultimate and Prime, there is another I can never remember), and to my knowledge only Prime also handles nitrate. The purpose of a conditioner that deals with whichever of these is present in the tap water is to handle the initial influx of a very toxic substance (in the case of ammonia and nitrite, less so with nitrate depending upon level). Conditioners like Prime are effective for about 24 hours, by which time the aquarium's plants and/or bacteria should be able to handle the residue.
Personally, I do not recommend Prime unless you have need for all of the above. Adding un-necessary chemical substances to an aquarium should be a last resort. According to many chemists and biologists, the chemicals in Prime likely have a reaction, and while the effect may not be readily apparent, I see no reason to risk it. The less "stuff" entering a closed system with live fish and bacteria, the better. Nature can handle all of these things; let it.
FlashGuppy, if you load a site that sells aquarium supplies online, such as Big Al's or Drs Foster& Smith, and others, do a search of water conditioners; they will group them and you will see the many brands, and they probably have info on what they do. If you know what is in your tap water, track down a conditioner that only deals with that.