Tactics Trainer vs Curated Tactics Problems
I've used Tactics Trainer quite a bit over the years, with my rating mostly varying between 1400 and 1600. Since starting the Seven Circles tactics training in July, though, I haven't spent much time on TT.
There is a significant difference between the tactics you'll find on TT and the problem set that I selected for my Seven Circles training. On TT you get completely random positions within a given ratings range (about +/- 50 points from your current TT rating).
There is no attempt by the computer to give you problems ordered for instructional value. For instance, you won't be given a 1 move checkmate, then a similar problem but converted to a 2 move checkmate, then a similar problem but converted to a 3 move checkmate, etc. IE, the problems are not curated and ordered by a trainer specifically for instructional value.
The quality of the problems on TT varies widely. It seems most are automatically culled from various online games (blitz, bullet, turn-based, etc) and then reviewed by a human moderator. I often find problems on TT that are very complex, requiring in-depth analysis of lines, but which are rated lowly because the "solution" is only the first move of a 4 or 5 move solution (or an even longer sequence). The downside of this is that many people end up getting the problem correct by simply quickly guessing the first move or making the first obvious move. But they don't understand what is really going on in the problem. They haven't worked out the lines. So the fact that someone got the problem correct in these cases isn't really measuring anything useful other than they are good at quickly making an obvious, natural move--that may or may not be correct.
The problem set I am using for the Seven Circles training is the first 1200 problems from the following two books:
These books are the first two in a five book series of tactics problems composed by several Russian chess trainers. The difficulty is designed to gradually increase from around 1000 FIDE to about 2200 FIDE. The first 1200 problems--the ones that I am using--increase up to a difficulty of about 1500-1600 FIDE, with the most difficult problems being 3-4 move checkmate and "move and win" problems.