Are Starter Kits Worth Buying?

If the contents of the kit are cheaper than buying the same items individually, then yes, they're worth it. Especially if you're starting out and don't have anything.

Ultimately, though, it's still a matter of getting what you pay for. Some items don't need to be expensive (nets, gravel, decor etc.) but you'll want a good heater and a good filter for the tank. If it's a starter kit with branded items, you can look them up online and see what the reviews are like. If it's an unspecified "own brand" model, then that doesn't always mean it's rubbish, but I can imagine some sellers bundling up cheap products to make a very cost-effective "starter pack" to lure in people, especially now that we're in the run-up to Christmas.

If there's a starter kit you're looking at, just look up some of the individual items online. You can see what they go for (and it can be cheaper to buy individually online than as a kit in a pet shop) and see what kind of reviews people have left. Then you can make an informed decision.

Assuming you want tropical fish, you'll need the following:

Tank
Light
Heater
Filter
Fish tank thermometer (just to make sure the heater's doing what it says)
Substrate (gravel or sand, depending on the fish you want to keep, one of these may be a better choice than the other)
Food (again, this might vary by the type of fish you keep)
De-chlorinator

These items may or may not be in starter packs, but they're certainly worthwhile:

Net (in case you ever need to remove a fish - or just to skim anything like dead leaves)
Some kind of syphon/vacuum, as they make water changes a lot easier/quicker
Spare bucket for water changes
Decor - live plants, silk plants (on the whole, avoid plastic plants as they can have sharp/pointy/rough edges that'll harm the fish), bogwood, caves, even things like castles and shipwrecks if you like this, the fish aren't fussy, but they want places to hide occasionally and it helps create a more natural environment if they've got things to swim around