I own a Umarex Octane. I purchased it this past April. I have put about 500 rounds through it, and I still don't like it!
Umarex told me, the accuracy gets better after about 1,000 rounds. Oh? Well, about every 10 to 20 shots, you have to retighten the stock screws and the cross bolt. If you don't, you can't keep pellets on a 25 yard target, much less in the 10 ring. Not wanting to go through that constantly, I put a groove in the stock so I could tighten the cross bolt with out removing the stock. I then used red LocTite to hold the screws in place. That helped a whole lot, but the groups average about one inch at 25 yards if I do my part. The most accurate pellets hover around 14 grains. Anything under 10 grains scatter like a shotgun!
The scope it came with failed after about 50 shots. Just as well, as the reticle is in the wrong plane, which means the aiming point changes with your eye placement. It is not serviceable (or worth it) in my opinion. I replaced it with a Stoeger 4x16x40 which has held up fine.
One caution. If you have to remove the stock, be very careful! The pins which hold the trigger, sear, etc., are held in place by the stock. Lay the innards on its side, and the pins fall out. Getting things back in place almost requires three hands!
And folks wonder why I like my Benjamin 397 so much!