In my opinion the 800 score tests are really off putting, specially verbal, and side track you from the actual GMAT aura. I'll break down my experience section wise.
Quant: Quant was not that much tricky but rather lengthy and calculative. If you keep getting questions correct, the test will offer you questions which are intensive in calculation or you may have to consider 'n' number of cases to get to the answer. I could never get a Q50 on 800 score tests even after getting 27 out of 31 correct, whereas I was consistently getting Q51 on Veritas Prep,
GMAT club tests & GMAT preps.
Verbal: There was a time when Verbal CAT's of 800score tests shattered my dreams of scoring >740 in GMAT (imagine your test being just a week away) as I was getting a score in the range of V32-V36, but somewhere deep down inside me I had a feeling that something is wrong with the questions. So breaking down section wise:
CR: Unusually worded. The correct option was almost every time illogical and out of the blue. FYI CR was my strength (with an accuracy of 90%) and I would get only 40% CR questions correct on 800 score tests.
SC: Most of time tested meaning based approach rather than grammar rules.
RC: Too lengthy. in 2 out of 5 tests I got 5 RC's with 6-7 paragraphs in each. The questions were also sometimes not worded properly.
In short IMO if you want to improve on your time management skills then go for 800 score tests because after giving 5 tests I feel that the only purpose of the tests questions was to not let you finish the test on time.