While searching for alternatives I found the HP 300s+ Scientific Calculator. We also used to have several calculators from HP which I also very liked. It looked like a decent entry-level calculator which is advertised with a "Perform decimal and hexadecimal conversions." feature. For 15€ including shipping directly from HP it looked like a bargain. It was delivered very quickly by UPS.
The first thing I wanted to try, of course, was the hexadecimal conversions as this was the main reason for buying the calculator. Unfortunately I couldn't figure out how to do this. Even the manual has no hints on it. So I asked the (german) HP support on Twitter for help, who referred me to the calculator support team. I called them, but the only help I could get there, after the support guy was asking his database or colleagues for several minutes, was that I should try to change the language (which the calculator does not seem to support) or to remove the batteries. After reexplaining that I'm searching for a promoted feature that isn't mentioned in the manual the support guy promised to do some research on my problem and call or write me back. I've heard nothing ever since. Next try was a post in the international support forum of HP, but after more than a week there haven't been a reply either. I'm not very pleased with this situation as this makes the calculator almost useless to me.
Nevertheless, the overall impression of the calculator is what you pay for. It handles all the standard functions like trigonometric, exponential and logarithmic functions and even offers some statistical functions. You can edit and correct your typed in expressions which is very handy. It looks like it can do even some analytical calculations as it e.g. evolves sin(π/4) to √2/2 (see picture), but this seems to be only possible for very few expressions. Never the less the 300s+ has a nice rendering of expressions and especially fractions. Sometimes the screen is little bit small and so you can not see both, the entered expression and the evaluated expression, at the same time (see picture as well).
The hardware itself is a little bit cheap. The buttons are the usual quality you expect from a 15€ calculator and cannot be compared with the very nice buttons on the classic HP calculators. A little bit confusing is the 'DEL' button as this more works like a backspace and therefor I would labeled it with the typical back arrow. Also the OFF is not on Shift+ON as I would expect but on Shift+AC which I had to search for. The viewing angle of the display is horrible. You can configure the contrast in the menu, but if you set it to be OK when looking straight on, you almost can not read anything when viewing at an angle of about 45 degrees. This is a real bummer and the second reason why I wouldn't buy this calculator a second time. Also the color of the display is more the traditional blueish on a green background than the black on white background as in the rendered promotional pictures. The 300s+ runs from both solar power and a battery. This looks like a good idea in the first place, but the display is hardly readable without background illumination when its dark so a solar power only version would suffice. The provided cover for the calculator is nice if you have to carry it around e.g. in your backpack. You can slide it under the calculator when using it, but then it acts like a seesaw which is really annoying.
So this calculator does not suite the excellent reputation which HP calculators used to have. If you need just a simple calculator for very occasional usage the 300s+ might fit you, but for anyone else who's searching for a more decent calculator it's not the perfect choice.
Update 2014/04/18: After more than 2 weeks I got a final answer on my hexidecimal conversion from the HP Calculator support by phone. After intensive research they found out that the calculator is not capable of doing this very complex calculation and I should buy a more advanced model if I'd like to use such a feature. After pointing out again, that the marketing brochures advertise this special feature on that calculator, they agreed that marketing materials must be wrong.