I have the Vortex Viper 4-16PST FFP and the 6-24 version. I'm very pleased with them. Love the accurate tracking, and the glass is acceptable, though I'm not comparing it to a NF. The 4-16 has been on my 260 Tikka for 3 years and has been carried in heavy undergrowth, tracking hogs, and I haven't yet accidentally moved the windage or elevation turrets. So, I have no complaints, only praise. And as for them being budget scopes, well, that depends on what you call inexpensive. I do have a minor gripe, in that the crosshairs illumination is too bright to suit me, even at minimum setting.
If you want a FFP scope with mil/mil, turrets and very accurate tracking, and with a great warranty, and get it for less than $1000, this is one of the few available. Great reticle too.
I don't have a problem with eye relief on the Vortex scopes.
I like the scopes, as you can tell. If I didn't have them or wanted to replace them, I'd look at the Burris equivalents. Even though I could afford it, I'm not stepping up to NF or S&B. I hunt deer, hogs, and coyotes and shoot a good bit of paper, and the Vortex is way more than good enough.