Update: Most bugs now appear to be fixed, and if you have any, make sure that first of all you aren't in the beta for Portal 2, since if you are it will CTD as soon as you get past the Valve intro video, if you have sound issues, rebuild the sound cache or do a clean uninstall and reinstall it for the fixed cache direct, make sure you have Portal 2 installed on the same HDD, and if all else fails, verify the integrity of the game cache on both TWTM and Portal 2.
Anyhow, onto the review of the actual game/mod now that I've had time to play it after fixing the bugs;
First of all, if this was a workshop submitted mod (not possible due to the extent of the gameplay and asset changes in some areas, therefore the store mod) it would most definitely be one of the highest rated community campaigns along with 12 Angry Tests and Designed for Danger. It may not be the strongest in terms of pure level design and perfection of the core Portal formula, but it certainly makes up for it through having the most significant gameplay changes to come out of the Portal community yet. If you're a fan of the Portal series, this is definitely a must play, especially considering it's free, you'd struggle to not got any sort of enjoyment out of TWTM, even if you didn't complete it or care for the TM mechanic much.
It doesn't have the strength of a proper retail game which is as to be expected due to the fact it's a community created mod by a couple of guys in their spare time. Most of the downsides amount to length of the mod and levels of polish. If this had been picked up by Valve for instance and given another few months of development time to polish it up and add to it, it could have no doubt been sold as a single player expansion to Portal 2 and have been worth paying money for.
In summary, if you're a fan of the Portal games, play it. It's free, you've got nothing to lose, and there's a very strong possibility you'll like it. Interested in seeing what the developers of the mod go on to do, since this shows a lot of promise from them in getting a job in the industry or creating their own games.