I've been seeing a number of people picking up the Sylvaneth models to play Age of Sigmare with, at least locally. Including myself. I thought I'd talk about some insights of mine having played them a few times and deeply contemplated their battletome.
First of all, if you play Sylvaneth and you don't have a few Citadel Woods (or a home-made version), you're really shorting yourself. An all Sylvaneth army needs Sylvaneth Wyldwoods to offset their slow movement and trigger a number of unit abilities. Alarielle and Drycha are the only models the Sylvaneth have that can move more than 7", and those are big gnarly one-of-a-kind characters. The Sylvaneth Allegiance abilities let you teleport your units around the battlefield, but only if there's Wyldwoods on the game board. Not only that, but nearly everything in the Sylvaneth warscrolls has something that either requires a Wyldwood to use or terrain. Invest in them.
The second thing is that the Sylvaneth have some pretty cheap wizards and some army-specific spells that make those wizards worth taking. The Regrowth spell in particular is an auto-include if your Sylvaneth includes any monsters at all. When those monsters are paired with a Branchwych or Branchwraith that knows Regrowth, you are forcing your opponent to commit enough resources to bring the monster down in a single turn. Because if they don't, they risk the monster healing D6 wounds. For most Sylvaneth monsters, that's up to three wound categories they can recover.
Now for some unit specific thoughts.
Dryads are pretty tricksy to deal with, especially if they're in a Wyldwood. They're extremely defensible in these situations if the unit numbers 12 or more; I'd run them in a minimum unit size of 20 just to have some time before they drop below 12 models. The fact that they can improve their armor save to a 3+ while in Wyldwoods is an surprise for something that's the rank and file unit for Sylvaneth. The other dirty secret is that Dryads have a 2" range on their melee attack. Sure, about 75% of the dice rolled aren't going to result in a wound, but when you count up how many attacks Dryads get after piling in, that's still a lot of dice.
The other bread and butter unit are the Tree-Revenants. I haven't quite figured out how to make them work yet, but my thoughts are that you need two units of ten to make them effective. I personally only have five and the only time they proved useful was in a team game where my partner had another five-man unit. It forced our opponents to focus attacks on one unit or the other. So I think two units of ten would let them start to become effective. I mean, these guys only have one wound each; they require some supporting units and large unit sizes.
The big ents (Treelords, Treelord Ancients, and Spirits of Durthu) are powerhouses. Even the Treelord Ancient will stomp things in spite of his comparatively lackluster weapons. A Treelord and Spirit of Durthu will both cut swathes through units, heroes, and monsters. The Spirit of Durthu is simply a monster killer; it does 6 damage if unhurt and it requires somebody to inflict 10 wounds to bring its Guardian Blade damage down to D3.
The Kurnoth Hunters have always caused problems for my opponents. Personally I run mine with Greatbows because my shooting is not so great otherwise. The 30" range of the Greatbows lets you have run of the board. At first glance the Greatbows may not seem like the best option since it means a fairly 'meh' melee attack. However, the Kurnoth Hunters can stomp units underfoot and inflict mortal wounds. They're also great to run with a Treelord Ancient as your army general because of their relay post type ability for command abilities. With their wound count and bravery, the Kurnoth Hunters are also very difficult to get rid of.
If you're going to play a lot of Pitched Battle style games (as outlined in the General's Handbook), you should take Branchwyches over Branchwraiths. The ability to summon 2d6 Dryads into a Wyldwood just isn't all that useful with the limitations on summoning in Pitched Battle games. On the other hand, the Branchwych's unique spell is offensive.
My final thoughts are on Drycha Hamadreth. She is a model dedicated to killing large swaths of infantry. She is also difficult to use since her biggest area of effect is the Flitterfuries option. Unfortunately, that ability affects both friend and foe. So far I've had more friendly casualties due to her than enemy casualties. I think the tactic to use with her (at least with Flitterfuries) is to use a Sylvaneth Wyldwood to teleport her into your opponent's back ranks and cause a lot of problems that way. The worm option is definitely the less risky option and most certainly meant to cut down horde units that she is in melee with. She is also a wizard, so she has access to the standard two spells and the Sylvaneth only spells. Her unique spell is also an area of effect, so use it wisely.