Both Combat and Watchman in amounts where I can have some feedback.
1. In PvP, people rarely look at spec of Sentinel. I see from time to time "OMG selfhealing is op" or "OMG roots and transcendence are OP" ("7k Sweeps are op" happen too, but it's for some reason mostly about guardians), but nothing pointing people feeling more strongly about one spec than other. Generally they either appear terrified by even thinking about fighting Sentinel, or mockingly say that they can easly kill it, but in a way "I'm skilled", not "I kill it in my sleep". Obviously, most people that don't have any problem with class don't voice their opinions. It seems Sentinels are considered dangerous because they will be focused fast, but not considered easy kill because they are rarely focused first. So class considered either strong or balanced, but certainly not weak.
For PvE, very good damage dealer I guess? Only reason why you wouldn't want one more skilled Sentinel in your raid is if you need more ranged DPS. Outside of properly organized group people seem to forget about Inspiration, are afraid to ask, assume Sentinel knows best when to use it, don't know. But since it's our best baseline group utility, I expected people to show more interest in it.
Combat doesn't have any special features people could notice in PvE, Watchman has his self and raid healing, but it seems to be rarely considered or noticed.
2. Combat - great burst and fun, good mobility(Speed increase in Ataru Form really makes difference in PvP) and PvP utility with Transcendence and roots, less of a dueslist due to lack of healing and main trick(Precision Slash) being easly countered with CC. My spec of choice for PvE due to most fights inclluding considerable downtime on boss, and damage mostly limited by CD.
Watchman - my greatest issue with that spec was when while questing, I was killing groups of standard mobs in arund 15 seconds, and refreshing Merciless Slash buff required delaying finishing off one enemy, because looting and making it in time to next group before buff dropped was impossible. It's a very unfriendly mechanic for fights with lot of downtime. Focus management is also harder, as you can(and if possible, should) use up 7 focus (with 8 required for it, 66% of total resource pool) in single GCD. Self and raid healing is great, but fact that single short downtime can mess up your huge nuke, or lag/ability delay cause yout DoT to fall off instead of being refteshed with extra stack(lags are still here, situation with ability delay got a bit better) makes it less fun for PvE, where bosses rarely allow mele to stick to them. It's good it's a bit harder, it's not good that you have so often build up all those stacks over and over again from 0 in PvE.
Both specs have not-so-usefull AoE. Back in a day when their focus cost(talented) was equal 1 it was worth using Sweep between other skills for some extra AoE damage, or spamming Cyclone Slash to interrupt capture, or just for damage. Now when you need to use 2 Strikes just to have enough focus for single Cyclone Slash, and still have to face enemy, it's very rarely used skill, so decreased GCD and cost on Ataru Zen is bit out of place.
Also both specs I consider 'duelist' for PvP - great at engaging enemy 1v1, both specializing in different kinds of fight - Combat for fast kill before enemy can kill him, Watchman for outlasting enemy by constant, steady damage, good ability to stay on target and selfheals. Some tool to set up those duels inside warzone(move enemy away from his group) would make this even more fun, but well, we've got tools to ignore his group(defensive CD) or CC for a while (AoE mezz), so it's fine.