I think the square nail tip looks beautiful and is excellent for a lot of nail art as you get a bit more surface area to work with, but for me it isn't practical. 1) Hubby hates square tips (not that that prevented me from trying them), as he feels they look unnatural (it's true the average unpainted normally trimmed nail will have a slight curve that follows the fingertip shape), 2) I can't sustain them as they tend to get caught on things, and 3) my natural nails are too weak, and the corners will bend, causing polish to lift (regular or even gel polish), and causing the nail to eventually break. So I tend to round mine off a bit. The weakness at the corners generally determines how much, and whether they end up being squoval or round. Usually they don't go as far as oval. Mine are usually in the range of 2 to 8 mm beyond the tip of the finger. 5 mm is quite 'long' for me, with my track-record, but I enjoy that natural-but-slightly-elongated look. When I was playing a lot of piano, I couldn't think of having long nails; career and activities can end up influencing certain grooming choices.
What helps me with the nail weakness is gel polish. (As would gel nails, I imagine.) Regarding the drugstore kits, I have the Sally Hansen one, and have used it with the SH and Gelish polishes. A mani will usually last for two weeks or more. I have one thumb right now where Gelish Sheek White is lifting in under a week, but as it's only one finger, I suspect it's my fault. I might not have applied a complete layer of Foundation Base Coat. They say 'thin' coats, but it still needs to be a complete, 'non-scratchy' layer (by that I mean not dry or partial or patchy anywhere on the nail; the brush should be lightly coated with base coat, not dry). Obviously, it will not work well for everyone - body chemistry can be a factor. But never peel them off, as you will be peeling off part of your nail with it, which will definitely make your nails more thin, brittle, and breakable. < I say this sheepishly, as there is that temptation to peel when you get a bit impatient with removal, or you're not by the acetone when a little edge has come loose, or when the first round of wrapping for 15 minutes wasn't quite enough. Resist! I showed the damage on one of my own nails in a photo in the 'nail polish you're wearing' thread. I try to let the acetone do its work.
I think nail shape is fully subjective. Do what you enjoy, and rightly what will keep you healthy.