I have mixed feelings about them. I think going to non-medical solutions is a good idea in many cases, and we've gotten so far from simple remedies and into complex, over-processed concoctions and of course the overuse of antibacterials and pharmaceuticals.
I don't agree that everything sold through a multi-level marketing or direct sales company is more expensive than what you buy in a store. Every store is marking up the wholesale items and paying huge staff and overhead and insurance and workers' comp. So they buy in quantity but then they mark things up immensely.
That said, I do get concerned when any company tells you that you have to buy one product for this and another product for that. If an essential oil company is telling you that you need 20 different oils (one for headaches, one for colds, one for stress, one for kids, one for hot flashes), then a) you have to buy a whole ton of stuff and b) you have to play "kitchen chemist" and pick out a different item or a different combination yourself.
I think there's more value in prevention than in trying to choose something after you have a problem and hope it will treat it (illness, focus, whatever). That's where all the research is done now in food science - prevention, repair of past damage, and so on.
In many cases, oils probably won't hurt much (except maybe your wallet). I would certainly ask any salesperson you are talking with to provide clinical data in addition to whatever personal testimonies they are giving you. Ask which research institutions they work with (not what studies they finance themselves), what additional clinical trials have been done, etc. There are ways to determine whether a direct sales company (if that's what you're using) is well-rated (company ethics, accounting, chief scientific officer's qualifications, etc.) and ways to determine where the products are manufactured. If their products can be sold in some other countries who have strict standards for food products, that's a huge plus. If your salesperson doesn't know and doesn't have sponsors or mentors who can help with this, walk away. If you are at a "health food" store that makes its money on selling you huge volumes of stuff they have marked up, and if you are talking to a clerk with no background in the field, walk away.
Any reputable company in the field will be able to provide clinical data (independently verified results from researchers and labs and institutions). You can also go to www.pubmed.gov to see if there are scientific papers or research being done on the specific oil you are looking at. If it's worth pursuing in research (such as for sleep, immunity, focus as you mention, or whatever other issues you are concerned with), that's a plus. That's one tool that people in food science use.