I find both MP-1 and MP10 really fun toys to play with. Play-wise the main downside with MP-1 is the way he doesn't hold his rifle very tightly... that's my biggest beef with that toy. But when weighing up the pros and cons of both MP Primes, I still find MP-1 comes out on top. But MP10 comes a close second and is a really great toy in its own right.

As for others...
+ MP Ultra Magnus: well, it's a straight up repaint of MP-1, which makes sense but I couldn't justify getting it, even for $80 at Target.
+ MP Starscream: the first one was really disappointing, felt more like a pre-built model kit than an action figure. The colours look nice in jet mode but dubious in robot mode. The subsequent Hasbro version improved on the colours and plastic quality.
+ MP Skywarp/Thundercracker: continuing to use improved plastic quality was nice, but still suffers from the inherent drawbacks of the original MP Starscream mould.
+ MP Sunstorm: this is the only version of the new MP Starcream retooled mould that I have, and they have definitely improved it as a toy. But damn its expensive RRP!
+ MP Grimlock: Nice but somewhat simplistic for a Masterpiece. The top half of the toy fundamentally feels like G1 Grimlock to me with most of the more MPish style engineering happening from waist down. The toy has some clever gimmicks and the laser rod thing is cool. As someone who's always preferred the G1 comics over the cartoon, I was delighted that they omitted the standard TakTOM accessories in favour of the crown. It is a nice toy, but not worth the $150 RRP IMHO.
+ MP Megatron: an ambitious toy, trying to make a thin gun transform into a chunky robot was never going to be easy. Admirable effort for sure, but the laboriously time-intensive transformation is just too damn complicated that it ceases being fun to transform.
+ MP Soundwave: Great toy... transformation is very similar to G1 toy, which is probably more indicative of just how awesome the original G1 Soundwave toy is. I paid $135 for my TakTOM version, which is quite expensive compared to the upcoming Hasbro release. FML. :/
+ MP Cassettes: very nice toys, but expensive for such small figures (despite their complexity). The backward compatibility w/ G1 is a nice touch, and the engineering of the figures is clever for what they are. Again, Hasbro version trumps out with price.
+ MP Rodimus: Got the Hasbro version, which is quite nice and addresses some of the flaws with the TakTOM version. The toy-accurate colours are nice, but the execution itself is lazy. But overall quite a nice toy.
+ MP Lambor: Disappointing and overpriced at RRP. The shoulders' inability to firmly lock into place is really poor for a Masterpiece toy and I wish that it had come with a rocket pack accessory (possibly with an additional clip that lets it attach to MP10 too).
+ MP Alert/Tigertrack: skipped after my experience w/ Lambor.
+ Movie MPs: I personally don't even consider these toys to be true MPs.

...none of the limited exclusive repaints have interested me, 'cept for Sunstorm.