Here's the message I sent to the email address they gave. Pretty much sums up my thoughts.
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I have played a session with an artificer character. We did mostly combat encounter, of course. Here are things that I think should be improved or built upon.
1) Short-rest prepared powers are a great concept, but are too sparse. Applying them to more powers would be, in my honest opinion, make the artificer a much more interesting class. Why? Because it makes it that much more distinct from the warlord and wizard (the two closest classes we have right now). Yes, this means the artificer might use his powers before the encounter and then hand them out to his party, but I'm not arguing for the artificer to have all his powers prepared in advance: I think putting a bit less then one option per "power level"* that is prepared in advance would be interesting, as you could build a character that is almost entirely based on these.
To compensate for the lack of direct attacks in such a build, perhaps give him ways to play even without casting new powers - perhaps "sustains" that act in new ways for the prepared powers. For example, you could have to prepare "Fiery infusion" in advance, but if you choose to sustain it (say, as a minor action), it might sustain the effect, or a part of it, for as long as you choose. since you will be throwing out powers, it could be an interesting choice between sustaining the power you just used or sustaining the one you used a round back.
2) I think his "Restorative formula" need to be re-flavoured (healing tea you spray on allies? seriously?) and changed a bit, so they don't feel so mechanically close to the warlord and cleric. Healing surge+1d6 is all fine and dandy, but it is starting to be overused. Perhaps change it to another type of healing; maybe give resistance X until the start of your turn, or something like that.
Now for the positive: what you did well, I LOVE. The mini-trampoline is inexplicably awesome. For that matter, his utility spells in general are very, very nicely done. Healing figurine, Phantom structure... Very polished, very fit thematically. However, again, I'd like to see more preparation: right now, even though the flavour says it's all because of his magical tinkering, there is almost no mechanical reflection of that. I think they should be models for some other powers: non-damage, but useful in any situation, including combat. Perhaps one more utilitarian power ever two or three "power level"?
Lastly, I'd like to direct you to this thread, on ENworld, that you might have already seen. it's filled with good ideas and feedback.
Thanks for the opportunity to playtest the artificer, and for reading this email.
* [Obligatory "It's over 9000" reference]