It depends on how good your Cleric is at forcing attacks of opportunity. If you had more buddies they could do things like Greater Drag and Greater Bullrush to basically throw baddies into the woodchipper, the woodchipper in this instance being you.
Your best bet is if your Cleric has any spells that can knock enemies prone over a large area, though I'm not familiar with anything that will accomplish that and a cursory glance at the SRD doesn't seem to offer any options. Altneratively, they can summon a batch of wolves (later dire wolves) to trip everything around you, in which case you can just can stand there daring enemies to try to stand back up.
Even then, it's hard for a two person party to corral enemies in such a way to make Combat Patrol worth it. Moving to hit one enemy will often take you out of range to hit another. I'd advise against it from an optimization perspective (there's just too many more useful feats out there to waste on this), but go for it if you feel like it fits your character concept and your Cleric buddy is game in helping you get the most out of it.
Here's what you're trading for the alertness feat if you don't take the abil score upgrade and instead take the feat:
- +1 to damage
- +1 to hit
- +1 to AC
- +1 to Dex saves
- +1 to Dex checks
- +1 to init
The question then, ultimately, becomes, is +5 init (net +4), no surprise and no advantage on attacks against from hidden opponents worth it.
The latter two features are very dependent on your campaign and DM. Surprise should be a fairly frequent occurrence on both sides of combat, but that still depends on how willing your DM is to allow it or stage his monsters to have them obtain it (and kind of how cautious your group tends to be). Monsters attacking from hidden is another one that really depends on your DM, you group and how well you guys are at nosing out monsters from the shadows.
So ignoring those two items since they are DM and campaign dependent (and thus can't be weighted objectively), the question then becomes, is +1 to Dex more or less beneficial over 4 levels than +5 to init.
For pretty much any character other than the Assassin Rogue, I'd probably argue that they should take the stat upgrade early, the feat at L8 and then the last stat upgrade. However +5 init has amazing synergy with the Assassin Rogue and you're going to get a ton of mileage out of it.
If you're already at 20 Dex (if you managed to start at 20, or 18 and are at L8), then there really isn't any stat upgrade that is worth taking over this feat (though there may be better feats). Rogues are skill monkeys, yes, but +5% in a single ability set of skills is not worth the +25% to initiative and other features o this feat. The only one that would be worth considering at all is Con for the extra HP, but it's really not that many and you're better of boosting your damage significantly.
Take the feat at L4 (or even better, play a human and take it at L1).
Best Answer
There is nothing in Alert that prevents you from being pickpocketed.
Surprised is a very specific state which refers to the beginning of a combat. Surprise is a state where a creature is unaware of another creature when combat begins, in this case the surprised creature a) cannot take an action on its first turn and b) cannot take reactions until after its first turn.
It has nothing to do with out of combat situations such as being pickpockted.