Each Battlelore specialist pack and expansion includes two or more scenarios featuring the new units included in that pack.
A mere handful of plays isn't really enough to have a well-informed opinion of all the ins and outs of new units in the system, but it may very well be all the player has to go on when he is deciding on specialist units for his Call to Arms armies or how to use them in any scenario where they may show up.
After four plays of the two scenarios from the Goblin Skirmishers pack, I do have some initial impressions of the new units introduced, with the caveat they may well be ill-informed and are subject to change.
The Goblin Skirmishers pack introduces Red banner foot troops to the Goblin order of battle with a pair of Hobgoblin Spear Bearer units. In addition, one scenario includes a unit of Hobgoblin swords that was in the base set but not included in any scenarios.
Red banner foot units are slow, but roll a lot of dice while fighting. Goblins mitigate the slowness a bit because they have the ability to "rush" into melee and move two hexes if they attack an enemy unit in close combat. Red banner units benefit the most from being supported because their boldness increases the chance they will get to "battle back" and in most cases they will roll more dice than their attackers. This is even more true for the spear units, which get an extra die when battling back. While potent offensively, supported red banner units are also an intimidating way to anchor a portion of your line and this is probably their biggest boost to a Goblin army, which otherwise seems to have trouble holding its ground. The biggest weakness of these units is that they are still vulnerable to a "Goblin Run" which can prove costly in both lost ground and lost figures. Given that I'm not sure how often they'd be worth selecting as specialists. I suspect that Goblin Rush doesn't offset Goblin Run in most cases.
The new missile unit in the Goblin Skirmisher pack is the Goblin Slinger. This unit's attributes have been extensively revised in the errata, and this discussion is based on the updated stats. Basically, a sling-armed unit gets to roll one more die than a similarly situated missile unit of other varieties. A green banner Goblin archer, for example, rolls 2 dice when it attacks without moving and just one die if it moves. A slinger rolls 3 dice if it doesn't move and 2 if it does. This is useful, as far as it goes, but the sling's drawback is that it has a very short range of just 2 hexes. This forces the Goblin slingers to be closer to the front than is healthy for a light green banner unit and they had a hard time surviving to the end of the scenario in our plays. One suspects that they may be more of a benefit to the opposition as "walking victory banners" than damaging to the enemy. Selecting them voluntarily would depend a lot on the potential battlefield. A battlefield with a lot of line-of-sight blocking terrain would be suitable because other missile units may have trouble lining up long-range shots and will have to move a lot to get into firing position, reducing their dice.
By far the most useful new unit is the Goblin Band. We haven't tried incorporating the Goblin drummers into regular Goblin units yet, but when fielded as an entire unit, the Goblin Band is definitely useful. Indeed, it changes the entire character of a Goblin army because now there's a good chance that the Goblin units will get to be bold, providing more chances for battling back and, even more importantly, reducing the frequency of devastating Goblin runs. We had at least one scenario where there were no Goblin runs at all. The Goblin Band helps all friendly units, not just Goblins. Tactically this means that units in a band-supported sector can deploy at the end of a line and still be supported to boldness. Normally units need to deploy into 3-adjacent-hex formations I call "tercios" in order to provide a mutually supported group of Bold units. With a band present, those three units can form into a 3-hex line while remaining Bold. And even a 2-unit formation can be Bold if it has a band in its section. This is an enormous benefit, especially if not using the battle savvy rules, and would make a Goblin Band a very useful unit to select if given the chance.
So in order of usefulness, I would rank the new units from best to worst as Band, Spear and finally Slinger. In my ill-informed opinion, of course.