I can't comment on the 5800 for comparison as I've never owned one but I have an X6. My daily work unit is a Blackberry STORM with the latest firmware (vast improvement) which I've owned for 1.5 years. So I am familiar with touch screen phones.
So far my X6 with the firmware upgrades has been stable. The Capacitive touch works great, very iphone like (compared to my son's). If you haven't used a touch screen, it may seem odd at first but with experience you can get very good with it. It has a nice feel to it.
The GPS is great, and this is coming from a seasoned Garmin user. While I'll still keep my Garmin 765T for my car, from a mobile or walking perspective this little GPS unit works well. It has both an internal receivier (no cell signal required) and A-GPS (signal required). The internal works well, thus making it a cost effective option, something I like as this is my travel phone.
The 16gb built in memory offers lots of space for music or other media. The camera works well as a cell phone camera for general photos. If high quality is your need, get a 12mp digital camera.
I love the Wi-Fi option. Again, another great feature for the traveler looking to minimize phone costs.
Build quality is good. I suspect the "flimsy" comments are coming from internet armchair critics and not real users. The plastic back is thin yet sturdy and clips on with 6 catches.
I'm not a gamer and my life isn't ruled by cell phones. They are tools that help me manage my day. I conducted an exhaustive search of phones and had no loyalty to any one brand. From a cost and feature perspective, the X6 meets my needs with low cost options that will help minimize my cell phone costs as I travel.
Please consider that my focus was on obtaining full features for foreign travel purposes at minimal cell carrier costs. Whereas I use to carry my Blackberry (business) cell phone (personal) a digital camera, my Ipod and my GPS, I can now carry one X6 phone. I also don't have to explain all those adapters and calbes to airport security.

If your needs are different, I suppose there will always be a better phone option out there.
Good luck.