To add onto what Col. Kurtz was saying, use of photo-etched parts also often requires removing existing details on your model (either separate parts or things molded into the parts). You will require superglue to place most of those parts, although maybe some that will not risk being knocked off during painting, decaling, weathering, can be applied with wood glue. Always heat over a naked flame the etched brass sheet before usng it: it will ease any folding, bending required. Wear goggles as tiny parts can easily damage your eyes!
Although I have used some sheets, and still have a few, most of the parts can easily be done with sheets of plastic, plastic rods, the lead from good wine bottles (gives you a good excuse to open up that nice bottle!), masking tape,... So, unless you do ships maybe (cause those damn railings are hard to reproduce!) consider the price of those etched brass sheets before you buy. Check what they offer and see whether you'll be saving that much time and effort by buying them. I think par of the pleasure of modeling is stretching one's abilities, beating the challenge.
Good luck.