ineligibles are another resource for people to tap into who don't have first cut at the guys they want (and in d3 you should ignore the ineligible tag). its definitely valuable, but there are downsides. in d2 and low-mid d1, you get 1 guy for 5 years, instead of 1 guy for 4 years, but you get the same production from him. the result is his average production per year is roughly 20% less than the same player who was eligible. so, they have to be pretty good, for it to be worthwhile.
in high d1, there is an additional hit. ineligible players are treated as their listed class, not their "actual" class, for EE purposes. so a guy who as a sophmore would be great but probably stay, as an ineligible, is probably gone as a junior, even though hes exactly as good as that sophmore. this costs you that all-important 3rd year and because his off year is split over fewer active years, his production hit as a yearly average is even higher. i just recently signed an ineligible at kansas because i liked him a lot and it was him or a walkon, but generally, i recommend championship seeking d1 programs avoid ineligibles. between the production hit, the EE hit, and the chance they don't even show up, its pretty hard to justify.