I was looking into this as well and after reading a crap ton of forum posts and ~ 100 hours of play testing I put the UCmonk at the top tier of martial classes and maybe THE top of martial melee classes, due to flying kick. Seriously this one ability which is a class feature!!! This abilities power and importance can not be over stated. Its ease of use, granting of mobility, and given ability to grant full attack virtually every round! All other melee martials have to have a spell which is a limited use resource, or need a long complicated feat chain and in some cases equipment to pull this off. UCmonks, as stated, get it at level 5 and can use it once per round and at lev 14 twice! Couple that with the base change to full BAB and the incredible changes to flurry and no other melee martial can keep up with the damage (sans a well built Paladin using smite but they can only do that so many times to a limited number of targets but the kicker is you don't have to use things like that to sustain high damage).
Where UCMs CAN fall down is they have no built in "to hit" bonuses like other martials i.e. Barbarians rage, fighters weapon training, rangers favored enemy, paladins smite, etc.
This can't be over come with the base class abilities but can be mitigated with fixes at relatively early levs with a relatively low gold investment in permanent buffs (enlarge person, Magic Fang, Greater) or UCmonk specific archetypes (looking at you invested reagent!)
All that said a lack of built in attack bonus is NOT the end of the of the world, in fact it is hardly noticed because of how flurry works now. Yes other classes may have a higher to hit on their first attack but they (in general) only get ONE attack at their highest BAB. While UCMs , with flurry, get 2-3 and with a little luck and/or planning by level 10 UCMs can get up to SIX on a fairly regular basis at their highest BAB before they even get to their iterative attacks, amazing!(Set your monk to puree indeed!)
Defensively (counting saves and AC, and ability to avoid damage ie evasion and greater evasion) they are still at least as good as any martial class with the possible exception of paladins and even pallys can't touch UCmonks touch AC, and they don't get evasion. With new options fighters can come close but have to use class resources that UCM don't plus they still don't get evasion. If you take barkskin as a ki power (and really it should be your first) you jump to the top of the heap very quickly.
UCM saves will still be among the best in the game, with class features (ie still mind and flawless mind) and especially due to being a WIS based class the low base will save will hardly be noticed and if you still feel the need there are always will save traits and iron will.
Just adding this all up gets UCM to at least a low to mid tier four before you even touch most of your other class features. Again amazing!
Skill wise they are not overly impressive due to only 4 skill points per lev and int being a low priority if not a dump stat. Despite this they have some very useful class skills: Perception, acrobatics, sense motive, stealth, and knowledge religion. Four if not all five of these have good if not great out of combat utility (though knowledge religion may lag a bit due to low int) they can fill spy/unseen infiltrator, scout, and adviser ("HEY I NOTICED THIS! or That dude was hiding something -") roles very well. UCMs are skill wise, no rogue, Bard, or wizard but they are not a fighter, paladin, or cleric by any means.
How high up the tiers the UCM goes from here is really up to how you build and how much ki you have and/or can regenerate.
Can you build a crappy UCM that is only tier five? Yes, but you have to try.
Can you be a solid tier 3 with the right build?
If we go by the definition of tier 3 as being *-Specialists that are capable of doing one important thing very well while still being useful when that one thing is inappropriate, and generalists capable of doing many things, but not as well as classes that specialize in that area. Classes occasionally have a mechanical ability that can immediately resolve an encounter, but this is a rare exception.-*I believe so, absolutely!
Will it be top tier three? Well that again depends on your build, but also the campaign, your play style, and what you think is more important, a top tier combatant with SOME non combat utility or overall general utility with bursts of martial awesomeness. If your definition is the former then yes with only Magus giving you a run for your money.
As a note: Assuming you agree, UCmonk is the only non full caster capable of breaking into tier 3 territory, as generally accepted. (Arguments have been made for paladins being tier 3, meh.) All others in the tier are able to cast up to level six spells and are full casters (ie their caster level is the same as their character level).
Think on that a moment and let it sink in.