I think anything is fair game, but consider what it is you are trying to accomplish. And have an answer to "Why do this?". To what end? What is the goal?
Life is too short and you can waste hundreds of hours trying to follow everyone - or be heard.
I would recommend backing into it this way...
What is the end goal? If your end goal is to be noticed by everyone that follows your competition then yes, this might make sense. But that's only one part of the issue. Once you have their attention, what is it you are going to do with it? Will the attention they give you be time well spent? Are they going to come away from the experience better for it? Will it help them to see you are better or smarter or more interesting or a better value than the others?
Because the downside of getting everyone's attention is that you might actually get it. Then what? This is where I would start.
What is the end goal? Are you trying to acquire new members? Sell more product? Create good buzz? Educate?
Let's assume you are going to reach into your competitors twitter audience and draw their interest toward your own. And let's also assume you are trying to sell your product (vs. only generating buzz). Then you might want to understand what the value-add of purchasing your product is. I might also research my competitions customers (and my own) to understand buying behavior. Specifically, what is it about the product that people like. Why do they buy this product category at all? Where in the product market are they putting there money?
Then, I would spend much of my efforts in Twitter speaking to that conversation. Your conversation would be about soft-sell persuasion. Not hard selling (because that won't work in this format) but about explaining how you know more, how your product solves some issue better, cheaper, more effectively with less effort for the customer. I would talk about service, features, the market, get granular - and be the expert in the field. Focus in, be open, sincere, and allow the audience to relate by allowing them to follow along - but then offering a more involved relationship (should they want it). Not just pushing them a product brochure PDF but instead, offering a format that offers pre-sale support to answer questions etc. Drawing them in.
In your conversation (on Twitter) I would have my market / product terms (primary and secondary) listed and use in the body text and subject headings because people are using systems like TweetDeck to set text / content alerts. This will help people filter/sift your content and it will help you.
Tie it all together...
Ask why you want to draw away an audience and then build a conversation that supports your business goal.
If you don't have the focus / goal you will do this and not have a clear path forward, miss the mark (because there isn't one) and you might not only look bad to a new audience, but you might waste lot's of time as well.
hope that helps.