First and foremost, a bot is almost always going to perform worse than a human when it comes to discretionary Technical Analysis.  The only exception would be from HFTs (which as I explained earlier, do not exist with bitcoin due to lack of FOK).  Even then, if it were possible to slow time down to where a human could trade at the same speed as an HFT, I'd imagine the human would outperform the algorithm.  We are simply better suited to dabble in fuzzy logic.

But any strategy, be it support/resistance based, EMA based, MACD based, arbitrage based, market making based, is going to generally underperform when you automate it, compared to a human.  The most important part of using indicators in TA is to know when they are invalid and when to ignore them.  They are not perfect, they are not magic, they do not tell you anything that price isn't already saying.  To write a winning bot you have to either have a ridiculously easy opportunity, or years of trading experience FIRST.  Writing a long-run winning bot (that is 1000+ trades at a minimum) is nearly impossible with no prior knowledge of trading, with only a few exceptions.

The positives are that they increase liquidity.  If you want to sell, you can sell.  If you want to buy, you can buy.  There are also scores of losing bots, people testing bots etc.  My bot had a bug where it accidently sold for $3 less than the current Bid for a handful of coins.  I made someone's day!

The negatives only happen when they are in an environment suited for them, like we see now on most major exchanges for the "mainstream" currencies.  There have been lots and lots of power plays on real estate near very important servers for exchanges.  Basically they've created an environment where they can claim they are only using data available to the public, but there's zero chance that the public could ever compete on the same playing field.  HFTs, by definition, are finding information about supply and demand that no one else can find.

But this isn't the case with bitcoin, but I fear it might someday be.  I really don't understand why the exchanges decided to follow in the footsteps of the traditional markets and include features like dark pools.  It is unfortunate that they are actually how modern markets work, but we had/have a chance to start over again, why are we just following in the same footsteps?  You're buying access to private information.  There was a thread a few days ago where a guy was admitting that trading via the darkpool on MtGox was cheat codes, and he had manipulated the public's inability to see the correct supply and demand, to make a profit, many times.  I don't ever blame someone for taking an edge that is legally and ethically available, but I can criticize the choice for MtGox to provide such an instrument, without any counter measures that normal Dark Pool markets have like FOK orders, or some method of reporting them.  In effect, MtGox has a much much worse system than traditional markets.

But anyway, bots aren't really going to have any net impact on price that speculators don't already have.  Price goes where the market decides, and bots are controlled by market participants.  So long as you understand that just being a bot gives you no inherent advantage, other than in doesn't need to sleep, you would have to conclude that their impact on price is non-existent, the market participants want price to go there, so it does!