Sora is generally acceptable for recreation and exercise for sure - I wouldn't go below Sora, but I just bought a bike with Sora components (a 2008 Specialized Sequoia) as my "leave locked up outside a store" bike because I thought it was the right balance between decent components - I wanted something that worked reliably but was as cheap as possible in case it got stolen (so I could easily replace it). I also cannot stand certain low end stuff (I hate the road bikes where you have to take your hands off the brakes to shift), so I thought Sora was about right. As someone else mentioned, it's going to be light years above the components on your walmart bike.
The other thing is that a lot of the bikes that come mostly with Sora components often come with a Tiagra rear dearailler (like mine does).
From Wikipedia -
http://en.wikipedia.org/wiki/Shimano
Road bicycle groupsets
For 2009,
road bicycle groupsets include:
- Dura-Ace 7900 (10 speed)
- Dura-Ace Track
- Ultegra SL (10 speed)
- Ultegra (10 speed)
- 105 (10 speed)
- Tiagra (9 speed)
- Sora (9 speed)
- 2200 (8 speed)
There are some drawbacks to Sora on my bike:
1. The shifter is on the brakes but it's a switch on the top, unlike with Tiagra and up where the shifter is a second brake lever that's flush with the actual brake lever. In other words, with Sora you can't shift while down in the drops. May or may not matter.
2. The rear Tiagra derailler shifts fine, haven't noticed much of a difference from my Dura-Ace shifting. My front derailler, which is Sora, has been rather annoying and has some trouble shifting. However, I rarely ever shift the front derailler.
IMO, Sora is acceptable and light years above the components that likely came with your Walmart bike. Sora would be fine. But if you wanted to make sure you got a bike that's going to shift reliably on both the front and back rings, I think Tiagra is the level at which shifting no longer improves above it by any noticeable amount.
Unfortunately, I know around me bikes this year seem to come with either Sora or 105 (with a rather large price difference in between). The good thing about Sora is that if you ride it enough that parts wear out (like the chain rings) they're both 9 speed to you could slowly migrate it to using Tiagra stuff.