These lenses are of limited use. Better than nothing and you can get some shots you'd never get otherwise. If you're not really going to miss the $$, it's ok, but it IS hard to use - in my experience.
Figure on a tripod and remote because this lens is going to be very sensitive to movement. You'll need long exposures because they don't let in a huge amount of light. You'll be operating in manual mode as well, so do some testing first to know what you'll need to do.
I found it hard to manually focus since most DSLR's aren't really set up for this - at least I found it hard to focus without the nice split screen help you have on older cameras - partially due to MY fading eyesight..... lol If you can see better, you can focus better...
Having said that, I used something very similar Opteka 500 with a 2xTC to shoot wildlife and my sons rockclimbing. Great quality? not really but I got shots I never could have gotten otherwise. But I spent a ton of time locating subjects, making sure NOT to breathe on the tripod......
Giving you an idea of what I was zooming on EXIF says this was at 56mm - I took this with my son's camera to provide context.
Can't give you specifics on this shot because nothing gets written into the EXIF file, but you get some idea.... I'm guessing this was at 1000mm... I was shooting at max most of the time.
It was really was hard to keep the subjects in view.... you'd lose them with the SLIGHTEST nudge to the tripod....
Below is at 300mm - with the 70-300VR lens
Below is with the cheapo long lens - I really found this hard to focus manually - or to keep the whole set-up perfectly still. This was on a not-great lightweight tripod, using the ML-L3 remote to trigger the camera. All this roadside at the Oxbow near Grand Teton's east entrance - near the loop road - with a zillion people trying to get their moose shot. I'm thinking that this is 1000mm total with the 2x TC that 47th street sells the Opteka with. again, no EXIF data........ after this experience, I gave up on shooting wildlife with this lens.... didn't seem like it was worth the tripod/remote set-up time.... besides there were lots of shots that were fine at 300mm or less
As a comparison.....My youngest got the shot below with a Canon P&S aimed
THROUGH a binocular braced on my monopod.........