I'm not a fan of the tape machine, but the synth engines are pretty good. Outside sampling there are 9 of them:
1) String: I like the core sound of this machine a lot. It seems pretty versatile with the paramenters. Not sure how many real sweetspots it has but the main sound is good enough for me and small variations thereof.
2) Pulse: This machine is a little baffling, I like the sound you get when everything is just in the right place, but that is about it. The only place you can go from there is "a little less good" which is can good in a way sort of like playing with a filter, but not getting a totally different sound. This machine also has a unison mode which sounds really good.
3) Dr. Wave: This is pretty basic, there is waveform you can manipulate in various ways and you hear what you see. It is basic but actually very necessary for the OP-1 because you can your basic sounds out of it. I have nice plucky "asian" type sound I made that I like a lot. It is fun to play with and the sound variety is good. The sound itself is not super great but good enough for what it does.
4) FM: Good machine, typical 4 operator FM, you can't do as much as on a DX for example so don't get ideas about playing with envelopes per operator (which is sort of core to FM...), but anyway bass is good on here and other sounds too.
5) Digital: I could explore this machine more, it sounds good and is simple to use and get sounds out of. I haven't bonded with it yet, but I think the potential is there. The parameters seem effect the sound a good amount.
6) DNA: This machine is useless.
7) DSYNTH: This is a tricky machine to use, I would not think of this as a traditional VA synth that you can get classic sounds out of.
8) Cluster: Nice machine, probably a few different sweet spots, there are 1 or 2 I like a lot, but others will likely find more (not infinite though)
9) Phase: Do not care for this machine at all. If you like the core sound it will work for you, but the parameters do little to change that sound. This is a pretty harsh sounding machine.
Anyway, these are just general impressions.