TLC is fine for many things, but I prefer LC-MS, even a simple, quick, gradient, for many things. You can much easier figure the rate of reactions, the possible structure of the impurities, and how pure your product is in 5-10 minutes. For most compounds, GS is just not as easy, practical, or useful, even with MS, as most GC-MS are poor at ionizing real compounds. It is fine for many starting materials, but few of my products will fly in GC, even with derivitization, which adds a huge unknown also. And TLC is great for simple, non-polar, aromatic compounds, but most sugars, small molecules, and many useful compounds don;t show up by UV or other simple methods. And quantitation is very hard to impossible. But remember that absorption can vary wildly, so don't compare peak areas unless you think the absorbances should be similar, I just hit that issue recently, where a tiny (by NMR) impurity looks huge in HPLC due to large absorption.