Offline Pages was one of the first apps I bought for my iPad. Everyone raved about Instapaper but I stuck with Offline Pages...until recently. Both apps offer roughly the same functionality and are each priced at $4.99 (although there's a Pro version of Offline Pages selling for $9.99 which includes the ability to save entire web sites, not just individual pages). Instapaper has at least one significant advantage over Offline Pages though: it lets you save pages in folders rather than just dumping them all in one bucket.
There are two features I'd love to see both of these apps add though. The first is the ability to sense when an article spans multiple pages and to grab them all. I can't tell you how many times someone has sent me a link to an article and I click the "Read Later" button on my browser only to discover later (usually when I'm out of a hotspot) that the link only goes to the first page. To avoid this problem you have to scroll to the bottom of the article in your browser to see if it's all on that page or not. If not, you typically need to use the Print option on the web page to have your browser render a version with all the content and then click your "Read Later" button. Why can't these apps scan the page and see if the telltale signs like "Page 1||Page 2||Page 3" links are there? If so, they could at least warn me that I'm about to only save a portion of the article. Better yet, they'd have the ability to find the Print option, render the full article and then save it off for me.
The other feature I'd love to see involves marking up articles and sharing them. I recently read a great article about the news industry that I wanted to annotate and share with others. I put it in Instapaper and the whole time I read it I kept wishing for options to highlight excerpts, insert comments, etc. How about we start there and add the ability for me to make notes to myself like this in the version I see in Instapaper?
But let's take it a step further and give me the option to share that marked-up version with my friends. Either within the Instapaper app or somehow in the version of the web page my friends load, I'd like them to see my comments about that article. Yes, this request is a bit more challenging than the multi-page one mentioned earlier, but it's quite achievable and something that would add great value to the offline reader app. What I'm getting at is the need to enable more social networking functionality with these offline readers; not just sharing links but including someone's additional notes about it. It's a great service to customers wanting to share content as well as publishers/authors of that content who would benefit from the increased visibility.