There are two parts to this question. What's defined as 'good' and what's defined as 'running'. Basically if you're hungry or tired you might want to think about using an energy product, or if you're running for longer than 90 minutes then you should be making this part of your plan.
The main point to note is that you shouldn't need an energy gel if your run is less than around 90 minutes. However, if you're say running after work and it's been 4/5 hours before you had your lunch then you might be going out on an empty stomach, this can often make you feel tired and sluggish as the body is looking for fuel where there isn't any. You can solve this by either having a light snack around an hour before you go out or by having a gel just before you run. If you do this your body will react better to this giving you some short-term fuel to run on. If you're planning on doing a longer run (90+ minutes) then you should be thinking about eating a good meal about 2 hours before you set off.
If you're running for longer than 90 minutes then it's probably a good idea to fuel accordingly and start to take on gels/bars/chews around half an hour in. This will stop your energy stores from depleting too much, and give you a better level of performance when you're out.
We're all different and our bodies react differently to energy products, just like food people have preferences on flavour, consistency and intake method so the best advice for any product that you choose to use is to try them and make note of how you feel after you do. Some people prefer very thick neutral flavoured gels whereas others prefer tasty chews. It's all about trial and error, much like you do with your trainers, clothes and routes that you choose.
If you're new to the world of energy gels, then a good idea might be to try a taster box with a few different types and flavours so you can see what works for you.