Overview:
In Germany, supermarket discounter Aldi, to be found in most cities and villages in Germany, offers a prepaid GSM/UMTS SIM card which runs under the name of "MedionMobile". In addition to reasonable prices for voice calls (12 cents a minute to all fixed and wireless destinations in Germany), the SIM card offers an attractive prepaid Internet access for low to medium or daily usage.
The SIM card itself can be bought in any Aldi discouter for 10 or 15 euros with 5 or 10 euros of prepaid balance already included.
Alditalk is using the eplus network, which provides HSDPA particulately, often just regular UMTS with 384 KBit/s and eplus also suffers from frequent congestions on their 2G- as well as 3G-network, which may result in low data throughput and in inability to place and receive voice calls.


Data Tariffs:
There are three alternatives:

• volume-based billing: € 0.24/MB, billed in increments of 10 kb. (APN to use: internet.eplus.de)
• monthly phone flat: € 7,99/30 days, bandwith throttled to 56 KBit/s after exceeding 500 MB per calender month, just for use with mobile phones/smartphones (APN: internet.eplus.de)
  
• dayflat: € 1.99/day, bandwidth throttled to 56 KBit/s after exceeding 1 GB per day (APN to use: tagesflat.eplus.de)
• monthly flat: € 14.99/30 days, bandwidth throttled to 56 KBit/s after exceeding 5 GB per calendar month (APN to use: internet.eplus.de)

Activation of Data Options:

• volume-based billing: activated by default
• monthly phone flat: call 1155 and follow (german) instructions
  
• dayflat: just use the appropriate APN: tagesflat.eplus.de
• monthly flat: send the following USSD-command and use APN internet.eplus.de :
  
  • activate: *121*004#
  • cancel: *122#
  • check status: *123#

Roaming Prices:

• In the EU: 0.19 euros per 100 kb block.
• Outside of the EU (including Switzerland!): 0.49 euros per 100 kb block.
• For details on prices and roaming countries see 4) and 5) of this document
  

How to buy Top up Cards:
In Aldi shops there are vouchers of € 15 and € 30 available. You can also use E-Plus Cash Codes, which are sold at supermarkets, gasstations, kiosks and even on vending machines (similar to those for cigarettes).


How to check and top up credit

check credit	*100#
top up credit	*104*(top up credit)#

SIM activation:
It is required to go online once and register with a German postal address (no ID card number required). The SIM card will then be activated within a couple of minutes.

Here is a quick walkthrough for those who don't speak German:
Go to https://www.medionmobile.de/registrierung_iframe.php (if the link doesn't work, go to http://www.medionmobile.de and click on "Registrierung" -> "Jetzt registrieren" -> "Neukunde".

Fill the form:

Anrede: Mr or Mrs
Vorname: First name
Nachname: Surname
Geburtsdatum: date of birth
Strabe / Haus-Nr: street / street nr.
PLZ: ZIP code
Wohnort: city name
Land: country
Festnetz-Telefon: here I entered my ALDI-Mobile phone number, but I guess you can enter any phone number
E-Mail-Adresse: email address
SIM-Nummer: it's the SIM identification number (not your phone number!). You can find it on the same page of the PIN/PUK codes, where you also found your ALDITALK sim. It's also called "SIM-Karten-Nr.".
Inhre neue Telefon-Nummer: your new ALDITALK telephon number (prefix+phone).
You will be presented with 4 checkboxes, only the last one is mandatory: "Ja, ich akzeptiere die AGB und die Nutzungsrichtlinien".
The first three checkboxes are an "opt-in" request for some spam from Medionmobile. You can leave them unchecked.


Misc:

• All services are open, e.g. web surfing, SMTP and POP3 works just fine.
• APN to use: internet.eplus.de for volume-based billing and monthyl flat or tagesflat.eplus.de for dayflat, username: eplus, password: eplus
  
• Medion Mobile is a MVNO and uses the German E-PLUS GSM and UMTS network. The UMTS network is 3G only (no HSDPA at this time). GSM is pure GPRS, no EDGE capabilities.
  

Links:
http://www.medionmobile.de/content2009/mi_flatrates.htm
Medion Mobile
Test on Martin's Mobile Technology Page
Dayflat Info (in German)